When I first started buying these clamps, the price seemed astronomical and I kept looking for more economical ways of doing the job. I never found one.
With a few of these, you can easily construct special rigs for lighting, backdrops, props and camera mounting. They'll grip practically anything -- square, flat or round -- and the result is immensely stable and rigid.
You'll need spigots or studs so that you can attach lighting units, camera mounts and other things to the clamps, or so that you can attach two clamps back-to-back and use them as an immensely versatile joint. (There's a selection of spigots and studs in the Amazon photography department.)