One of my biggest frustrations with the kneeling position is the amount of tension that gets put on my shooting side ankle (which I have a long history of rolling/spraining). I can hold a solid kneel for a bit, but not over any significant string of shots. Earlier in the year, I saw a tip from Olympic shooter Matt Emmons about putting a supporting sandbag under the shooting side foot- but I never actually tried it. Another competitive shooter (US Air Force smallbore shooting team) suggested the same, but I still didn’t try it.
I couldn’t tell you why I didn’t try it, but my guess was that it looked like another piece of gear that I would have to carry around. in any case, I recently took my Triad Tactical wedge bag (which I usually use under the stock for bipod shooting), and happened to kneel on it. What a difference! The uncomfortable flexing of my body weight on that gap between the ankle and the ground is gone- transferred to the beanbag.
So, in short, if you want to dramatically increase the comfort and stability of your kneeling position, place a small sandbag/beanbag under your firing side ankle.