This was designed on FreeCAD.  The mount is simple. The octagon tangential diameter is 14 mm, and the spacing is 100 mm (4 spaces on Multiboard). 

I print it on it's side with tiny supports for the two pins that hold the charger in place to get better strength, but probably not necessary unless you are really rough plugging in your batteries. 

The octagons may need to be adjusted slightly if your printer has different tolerances than what I'm using. I use a BambuLabs P1P with 2 mm nozzle, and PLA.