My original packs are really old. Instead of buying new hand drill or new packs I gave 3D printing a try.
I had a 4S A123 Cells pack handy with standard XT60 plug - so I soldered myself a wire lead that converted XT60 to tabs + extra XT60 for charging using my charger. But the box is big enough to accept 3S 4000mAh packs as well. This will give plenty of space. If necessary you can just cut/slice the pack for your needs. Or scale.
You need to buy yourself car connectors 6.3 mm wide - long.
The way you connect your batter inside is up to you.
Everything is connected with M3x10 c-sunk bolts - 8 pcs (4 insert, 4 back),
1 x M3 x 20 for XT60 holder
4 M3 nuts to attach insert
2 car wire connector
The pack sits firmly in the socket so there was no need to install the locking bracket. So my printed version is not equipped with it.
The back cover is actually 2 piece merged to hide the sliding clamp opening. If you want to have the working clamp that holds the pack against springs then in Slic3er click the button split. And then delete the cover opening. I've printed the clamp but after first assembly I found it not needed. Also my A123 4S pack was too big to fit the clamp inside :( But with smaller cells (3 or 4S lion) I think it will fit nicely.