I scored 8 sets of these “double bar” clamps on craigslist for $40 about 8 months ago. You are supposed to use wood as the rails and it sandwiches your workpiece while edge gluing panels to keep them all flat. However, the rails will bow inward when pressure is applied. This is compensated by cutting a taper on the inside faces of the wood clamping rails per instructions. This did not appeal to me so I decided on 11ga. 1 3/4” square steel tubing. If I were to make them out of wood, I would want a decent hardwood, which would cost near the same as the steel. So being a machinist, I knew I could make it stronger and here it is. The tubes are cut to 48” and I can clamp up to 36” between the pushers. I tapped a series of 10-32 holes so the 12 position brackets can be moved along the top rail depending on the width of my workpieces. I can clamp from 1/2” thick up to 3 1/2” thick. The goemetry of these clamps are a neat idea, the more you tighten the screw, the tighter is squeezes the rails together, keeping it all parallel. I think Rockler used to sell these sets, but I havent seen them new anywhere .
-- I haven't had this much fun since hogs ate my little brother. www.rockybluewoodworks.com