I've got a few years wearing a thigh holster (SAS MK V HOLSTER - AIRBORNE) and I must say I really like that holster and setup.
http://www.eagleindustries.com/mainprod.asp?scv=0
If you’re getting a lot of debris while crawling you can adjust the holster so it is further back on your thigh thus eliminating the scoop problem. It is a fast draw combat holster and one I would take anywhere in the world (and have), on any mission.
The only thigh holsters I do not care for are the kydex holsters, too big, noisy and very unforgiving when you land on your arse……
My .02