Over-ear Bluetooth headphones are quite expensive so why not make them yourselves?
I've browsed thingiverse and found nothing which I liked. However the Duli v2 had some nice features I could use (thing:674179).
I've started out with some testing and finally had the right cups for the sound.
The current version works quite nicely for me.

I placed this as a 'work in progress' though at the moment it's quite finished.

What do you need:
1: PETG filament (I use eSun)
2: A bluetooth AUX Audio Adapter - I Used: https://www.banggood.com/BlitzWolf-Bluetooth-V4_1-Car-Handsfree-Music-Receiver-3_5mm-AUX-Audio-Adapter-p-1051628.html?cur_warehouse=CN (about €15,-)
3: 3.5mm stereo male jack plug - https://www.banggood.com/3_5mm-3-Pole-Golden-Male-Repair-Headphones-Audio-Jack-Plug-Connector-p-940438.html?rmmds=search&cur_warehouse=CN
4: 3.5mm stereo female jack chassis mount, it must have a nut for fastening it to the speaker-cup) - something like: https://www.banggood.com/5pcs-PCB-Panel-Mount-3_5mm-Female-Earphone-Jack-Socket-Connector-p-1038048.html?rmmds=search&cur_warehouse=CN
5: single string stereo wire, 50 cm in length and a 1.9mm in diameter (smaller is also ok.)
6: 2 Speaker drivers 40mm - I used: https://www.banggood.com/30MW-Replacement-Earphone-Speaker-Unit-for-Bose-QC25-Headphone-p-1232965.html?cur_warehouse=CN (€3.26 per pair) In my experience 40mm drivers are the best size, smaller ones wil give a flatter sound. (but if you really want to use smaller ones then just resize the inner diameter of the front baffle inserts.)
7: velvet ear pads: https://www.banggood.com/Velour-Velvet-Replacement-Ear-Pads-Cushion-For-AKG-K240-Studio-K240MKII-K270-K271-K271S-K272-p-1066665.html?cur_warehouse=CN (€2.74)
or
PU leather (better bass and privacy) ear pads : https://nl.aliexpress.com/item/Replacement-Ear-Cup-Pads-Earpads-Cushion-For-AKG-K240-K241-K270-K271-K272-K280-K290-K340/1235381384.html (€10.64)
8: When using the headphones wired you'll need 1.5 mtr cable with at least one 3.5 mm stereo male jack and whatever you need on the other side. (I had an old broken USB headphones lying around, that works also making the headphones very versatile).
9: Rubber headband padding
10: A soldering iron and some solder.
11: All the printed parts, printing the baffle and baffle insert twice.

Homemade Prusa MK2 from Tevo Tarantula parts

No

Yes

.25mm

15%

I used PETG form esun because PETG has practically no shrinkage and is somewhat flexable.
Therefore I haven't added any tolerances for other filaments like PLA for instance.
Also, don't print to fast max 45mm
Make sure when adding supports you don't add any to the holes in the left and right headbands where the wire has to pass through.

To me it looks straight forward, but hey I made it. :)

Use the following sequence:
1 Handbands (left, right and top headbands, top-gear and top-gear pin)

• Make sure the headbandtop inside is cleaned out completely from support material.
• Remove sharp edges from the left and right headband
• push both right and left headbands in the top headband making sure the left and right headband can pass one another. If this is not the case, just slice a bit off from both left and right headband ENDS on the top side where the theeth are. keep doing this until the bands can pass one another.
• Insert the topgear pin as far as it goes into the topgear
• Insert both left and right headbands in to the top headband making sure there is room on top where the topgear must be pushed in. Adjust both bands equally so there's room and the teeth align) , Push the topgear in, wiggling and rotating it a bit until it sticks.

2 the right speakercap: (Buttons, BT device, right Speaker-cap, right Headband, Front baffle, baffle insert, driver, cable, 3.5mm Male jack plug)

• Insert the Plus, minus en play buttons (make sure these move smoothly in the holes)
• Insert the bluetooth device making sure you have the jackplug-side up. (Before you do this, make sure the BT box contains no residual support material.
• solder 2 wires of the 3 wire cable (right channel and ground) to one of the speaker drivers (don't overheat)
• cut the wire at about 8cm from the speaker.
• press this speaker driver in to a baffle insert.
• combine the cable from the speaker (Right and ground) and the cable which has been cut and solder them to the male 3.5 stereo jack. (all 3 wires)
• push the jack plug into the BT device.
• press this speaker driver in to a baffle insert without real force. Sometimes the speaker has some residue on the outside diameter making the fit worse. Because this residue protects and hides the internal speaker wires, very very carefully scrape some off so the diameter of the speaker driver has no bulge.
• insert the other side of the cable through the speaker-cup and then take the right headband and insert the cable through the holes. The speaker-cup hole and right headband hole for the cable should line up. (Make sure the blue-tooth plus and minus buttons face downwards).
• Press the speaker-cup in to the right headband (Make sure the hole for the speaker-cup ball in the headband has no residual support material).
• Take one of the front baffles and push the speaker insert with the soldered speaker into it.
• Push the Front baffle into the right speaker cup.

3 the left speaker cup (Left speaker-cup, left headband,cable, speaker, front baffle, baffle insert, 3.5mm female jack chassis)

• insert the cable through the left headband holes from the top and outside in.
• insert the cable through the left speaker-cup from the outside.
• adjust the length of the cable so the cable can be pushed in the groove on the underside of the top-headband and make sure you don't cut it to short (8cm inside the cup)
• Cut another 8cm cable and combine both wires and solder them to the 3.5mm female jack chassis mount.
• solder the other side of the 8cm cable to the speaker driver. (making sure you have the left channel connected and again don't overheat!!)
• place the 3.5 female jack in the hole on the bottom of the cup and fasten it with the nut
• Push the speaker driver in the last front baffle
• push the front baffle in the left speaker-cup

4 Finishing touch

• Force the cable into the groove on the underside of the top headband only in the middle part, because there the groove should be completely open.
• place, glue, stick the Rubber headband padding over this grove along the complete inside length of the top-headband.
• Now test the headphones, making sure the channel configuration is correct and the push buttons for the BT device work correctly, then place the ear pads.

I used some kitchen paper towel inside the left and right cups to dampen the sound reflection.

If your outer ears hurts after some time, (mine did) you can reverse the front baffles giving your ears 3mm more space (however this will diminish the sound somewhat).

Added new left and right speaker cups for using the headphones without BT, just using an Y-cable.
Another option with this is using only the right speaker cup for y-cable and connect the right to the left one via the top headband using the female 3.5 chassis jack in the left speaker cup and again use it wired like with the BT version attaching a double jack plug cable or an USB cable.