When I decided to start colour printing again about 8 years ago the market was flooded with second-hand Nova processors, so I bought a 12"x16" for about $100/£50. It would be hard to build anything similar with such good temperature control for the prices they are selling for now.
Keep watching ebay. I have picked up a 2-slot 8x10, 4-slot 8x10, and 3 "tube" 16x20 in different transactions over the last 6 months. Total cost including UK shipping about £120.
Several more have been offered in that time, but I hung on for the cheapest prices.
A 4x8 sheet of 1/4" ABS really isn't that expensive. It's easiest to cut with a table saw set-up for large sheets, but a circular saw or router, clamps and a straight edge should work ok as well.