Personnaly, I use bed sheets for my 8x10!
They're cut to 6x4 feet, one black, one white. Sewn together on three sides and turned inside out before sewing the last edge. To easily locate the middle, I have a 10in long red stripe along one edge which I place over the camera - easy to spot, easy to set up. I clamp the cloth on either side of the camera with large "paper clips" and I have two strips of velcrow sewn-in just under where the camera ends up being. I get under it and tie the two strips of velcrow and I'm in business.
Use sheets that have a tight weave count and ones that feels good to the touch without being slippery (forget silk). And. of course, ones that you can not see light thru ;-).
Easy, cheap, and works well. Heck, I can use when I need to take a nap even.