Firstly, not sure if this is in the right section as I cant seem to find the useful tip section, and secondly not sure if this has already been tried or not!
I was trying to think of a simple way of supporting the rear of the stock without using or carrying a cumbersome rear bag whilst stalking. I came up with an idea tonight of cutting a 7" (18cm) section of silver pipe lagging and wrapping it over the last section of the sling just in front of the sling swivel. It seems to work a treat giving quite a good grip for your non-trigger hand and a large amount of support which can be raised or lowered as necessary without any loss of strength. Also does not interfere with carrying the rifle. Mine was wrapped in webtex tape for a little camouflage! I'll test it out and let you know how it performs in due course.
Hope you find it useful.