the very best is if you can find a fallen tree that is about 10" in diameter and suspended above the ground about 18 inches. Just sit with your feet on the ground banks of your knees on the trunk, rear in the air and go.
Also if you have an e-tool style folding shovel you can go in comfort anywhere. Dig your cat hole, angle the blade 90 degrees and lock it down. Use it to support one cheek with the handle braced near the edge of the cat hole. Very stable, comfortable, and sanitary without being beholden to the availability of trees.