sling. Learn how to use it. Its quick and does the job. Offhand...learn to grab what you can or just deal with it. I wouldn't carry anything just due to an offhand shot.
That said we often use a walking stick or cut one...
Prone, for longer shots, I"ve used my wife as a rest, a backpack, or just a sling.
Exactamundo. I agree with Rost. I'd highly suggest shooting/practicing in field positions. If having difficulty with offhand shooting, just practice more. A tight sling helps a lot as rost pointed out. Learn to do it right, learn to use your surroundings like trees or sapplings. If you can, get down in a sitting position and learn how to anchor your elbows. A good buddy of mine practices field positions more than I do and can consistently shoot sub moa groups (at 100 yards) from the sitting position. The last time he shot my 9.3x62mm in this manner, he shot a .750" 3 shot group in the sitting position. A lot can be done with a certain amount of practice, no need for bi-pod or shooting stics while hunting elk. To be honest, most elk are shot less than 100 yards away anyway and that's an easy snap shot offhand for most practiced hunters... Since the OP specifically stated "400-500 yard shots across canyons" though, I'd suggest practicing from a pack and leave the extra weight and clutter at the sporting goods store....
