Exactly, having everything in there loosely wasn't the thing I wanted it to be. Plus, some kids saw keys, wallet etc. and were really dissappointed. I used very cheap press board (don't know if the translation is correct, sorry!) for the covers. It's 0,3 cm thick. For the "walls" I used maple shelf-board wood, we just had lying around at home.
I guess, you could do it better, but I'm satisfied for being this a spontaneous, one-day idea :-D