I've used plywood for enclosures before, but I want to use plastic instead so I don't have to worry about sealing it, painting it, etc.

HDPE Plastic like used in vision cages can be bought in 4'x8' sheets, and seems perfect - except I have to order it, and it's really expensive!

Does anyone have any advice on alternatives or where to obtain sheets of opaque (preferably black) plastic that can be screwed into, is thick enough to make into a tank / box, etc. I'd really like to be able to pick something up at home depot or something so I don't have to ship oversized sheets like this, but I'm not sure that's feasible. I could get multiple of smaller sheets, but this doesn't appear to save money.

Any ideas? In a perfect world, I could run to Lowe's and grab a 4'x8' sheet of HDPE that is 1/4" or 1/2" thick that is black in color for $40, but this doesn't appear to be possible.