Moldmaking Mold box materials

So far I've seen mold boxes for casting being made out of acrylic sheets, plywood and legos. However I'm going to need between 5 and 8 tiny boxes (with the longest edge being 10cm, and most between 4 and 7), so I'd figure the easiest and cheapest solution would be 5mm thick cardboard. Is this a good idea? Will it soak up the moisture from the silicone? Maybe there's something else that can be used for box building?

I considered making a one big mold for all the parts, but they differ so much in size and thickness I'd end up using twice as much silicone, risking falling short... Though honestly it still might be better than having to cut out 40 small pieces with a fretsaw

EDIT: Okay, seems like I found the answer after all, as seen in this tutorial, Oomoo hardens normally in a cardboard box, so that's a big relief.