Galco SOB
I've been using a horizontal middle of the back holster made by Galco, the last few months, and I must say, I like it. It wears more comforable than I thought it would, with the full size G21.
Its strapless with a tension screw. They're at Galcoholsters.com
Galcos cover six crossdraw holsters are nice too. I picked mine up locally for cheaper than Galco sells them direct. Check em out.