Collars? Yup, I didn't know what they were called either. I just wanted to show you guys what I got from American collars to mount stuff on my D90 with.
Their base pricing was less than equivalent collars at McMaster-Carr ( http://www.mcmaster.com/
) and they do customizations in low volume for a reasonable price.
I had them make a few for me with a flat edge and a threaded hole in the flat edge to make them more suitable for mounting things to them. I took some pictures and showed how I mounted my fire extinguishers using them here: