Here's another home-built one. It's of a down-draft design with a Dayton squirrel cage blower below the expanded metal stage. It takes a standard 14x25 furnace filter which slides between the expanded metal and the head space above the fan. I've attached a file given to me by Klaus Raddatz who was a regular over on the Hobby Heaven (now Spotlight Hobbies) board. It contains pretty much everything that you need to build a booth to industry specifications. For about $100 (back in 2004), most of which was the cost of the blower, I've got a commercial grade set-up that's given me years of trouble-free service.
We've got a big ole sheetmetal brake at work, so I plan to eventually go that route on my next one.