CR2032 (and smaller) battery holder insert for Bierkiste

Print the empty Bierkiste and this insert. Two of them stack on top of each other and fit inside the Bierkiste. Holds up to 20 pieces of CR2032 or smaller (CR2025 etc) coin cell batteries.

Not very print-optimized yet: the walls could be thinner to reduce print times. Whenever I get around to printing more of these I'll reupload an adjusted version.