Boris I know it is not as comfortable, but maybe you could try just taking a knee instead. Another thing to try is to sit with one leg straight(your left if your right handed) and the other bent at an angle to your right(if your right handed). This may not work on flat ground, but on a slope it does for me, I can actually shoot this way although not in as many directions due to my legs.

The Tazman aka Martin Price
Founder and President of
Virginia Disabled Outdoorsmen Club