Another idea I have, but have not used it yet: assuming you are hunting a long straight field or lane where you don't have to maneuver much: put a 3 foot or so cleat on the left and right walls at your desired height (say, 5" below window ledge). Then carefully measure the distance across the shooting house between the walls. Cut a 2x8 or 2x10 to this length. Rest your shelf on the cleats. Now for use, you can push it all the way forward toward the window when entering and exiting the house. When you are all set up, scooch it back toward you to the desired distance where your rifle butt naturally falls on the rear bag to place point of aim just off ground level. If you are worried about sound, cut a strip of carpet and place on top of cleats.

Someone do this and tell me if it works! haha