They need to be about 20cm wide and 20-25mm deep, with each a minimum of say 10mm height above the division between the two halves.
The idea is that you drill a couple of holes in the upper box and split the lower half into four compartments and put e.g. damp cotton wool into two diagonally opposite compartments to make them damp, dry cotton wool in the other two. Then you put a layer of cotton muslin over the top, held in place and stretched taught by being pinned between the top and bottom halves of the box - in the picture the grey oval is the layer of muslin. You drop some living organisms into the box, e.g. woodlice or maggots, then shine a lamp onto one wet and one dry part, and keep the other half shaded. What happens is that you get the animals moving to the wet/dark bit, of course - sounds obvious, but it's something that the kids need to prove.
The trouble is that a kit of five of these boxes costs about £50; what I'm looking for is some containers I can adapt to make my own. Basically a round plastic container, not too deep, at least 6-8" wide, with a clear top. Any suggestions on sources, or something that comes in a box like this that isn't too expensive?
Later: Full details of how the experiment works here which show why they are built the way they are.