Best I've found for woods carry of my Glock 20 is the Bianchi UM 84/92 Universal Military Holster in a cross draw on my hip. In this fashion the gun is completely protected from larch needles, mud, and rocks even if I am sliding down a hill on my butt. It is completely secure as well, and quick and easy to get to. Also, in seconds it can be removed from your pack belt and placed on the belt of your pants or anywhere your back or chest pack has webbing for that matter (not sure why you would want it anywhere else though if your chest is already covered with your binoculars and a range finder).
Do you know if a G20 with a 6" barrel will fit in that holster?