eggcrate is always, at every home depot or lowes I've been to, on a low shelf at ankle level. Not sure why, but it is. It's usually in the precut sheet goods isle, not with lighting.
I would imagine you'll only need a single sheet... building a box with it is the way to go.
and here's the secret to working with it: cut it with a screwdriver. You can use snips to break the pieces, but if you need to cut a straight line, the easiest, fastest way is to stand the sheet on end and push a screw driver down real hard, breaking the bits between the piece you want and the piece you don't want. it takes so much work out of it.