Ben, we built a riser a few years back to take to our own band's shows, but we didn't rent it out to others - much like your situation. We used 2 4x8 sheets of plywood, industrial carpet (just your typical blue-pattern berber carpet), and some 2x4's around to make a rudimentary frame. At first, we got about 40 milk cartons, put it up on those (it was surprisingly sturdy) and hung a curtain around it so no one could tell how shoddy it was
after that, a buddy of ours who scraps metal found collapsible stage pieces that were shot to hell (from and old high school or something), but we took the legs and bolted those on. Still sittin' in the trailer! We haven't used it in a while, but the collapsible legs really do help. I'd never rent the thing to anyone, though. Look into getting some from that link that Eric posted.