My battery for my mini excavator isd under the floorboard of the machine. I used the kit from Amazon (shown in pic) to move the terminals to allow me to charge, jump, etc. BUT, the relocation kit is such that you can short charger clamps together so I added an insulating barrier between the posts.

I noticed that the only indication of polarity was the plastic cap over the connection post. So I split the model in two using a dovetail to connect them. print one red, the other black and slide them together. only 0.1 mm of clearance so it's a stiff assembly.

I made it strong, overkill really, with 35% infill and 6 top layers/walls. 

Filament is PLA

Printed on an Ender 3 S1 PRO