I would like to recommend the Black Jacket.
I have one for my 8x10 and it is great. Light weight, comfortable to work with and it blocks the light from below as well as top and sides.
Since I purchased mine it has been improved so that on a field camera the light does not come through the space between the bed and the back.
For smaller cameras I use and have my students use black T-shirts which also work well, although somewhat porous.