Welcome to the forum. You will need to build a frame to attach the plywood and everything else to. For the size of box you talking about you could use 1x3 furring strps. Your cut list would be 4-4' long 1x3's, 4-46 1/2" long 1x3's and 4-22 1/2" long 1x3's. Build your two 4' by 4' frames and then attach them with the 22 1/2 boards.
I would recommend anyone doing these types of projects use a drill and srcews not a hammer and nails. Drill pilot holes first so you don't split the wood or use heavier lumber. Here's a couple of pics of a flood and drain table I built if your interested. VV