A few issues ago in RCM (march 2001, page 26) David Anderson wrote an article that talked about a "two pass mothod for forming crystal clear canopies". He said that he used an aluminum cake pan for his vac-box. Sounds cheap to me. Get one that has a flat flange around it, the kind that has a matching lide that slides on. Quite honestly I'm not sure if they make that type anymore, I was considering stealing my mothers.
The reason for the flange around the outside is that it will allow you to clip/bolt/screw the perforated top to it. Just make sure that it is sealed. Cut an appropriate sized hole in the side for the vac source plug your shop vac in and make sure it seals well and there ya go! On a larger unit you might need to put a brace across the inside of the box to keep the vac from collapsing the top. The Electronic perf board sound perfect too!