I'll pass on the advice given to me (from a long time High Master). One handed isn't the answer... as he said God gave you two hands - use them.
Since the important fundamentals of shooting are Stance, Grip, Trigger Control and Sight Alignment he reminded me that the exact position wasn't the real factor. His suggestion was to give myself the most stable "stance" possible... for a short legged, short armed fellow with a bit of a belly, the best possible seated position was to sit straight up, both legs extending out and slightly apart for maximum stability. Lean forward at the waist just slightly (not too much or you'll make it difficult to breathe... just enough to reduce back muscle strain and swaying). Essentially it's the same as if you were standing unsupported but your bending your body at the waist to sit.
When he first suggested this to me I thought he was crazy. I couldn't see how you could shoot accurately 50 yards in what amounted to a standing unsupported position. With a little patience and practice I found that it worked better than anything else I ever tried. The secret (as with all the positions) is, given a stable base or stance, the grip, trigger control and sight alignment be as perfect as you can make them. There will be a small amount of "arc of motion" but this is well within acceptable limits. If you can control the trigger and keep the sights aligned you'll find you can shoot consistent 10's and x's from this seated position.
I found I took a little longer (during the stage) using this method of seated (to let my arc of motion settle in and to allow me to really concentrate on the sights) but the results were amazing and I simply went a touch faster during the final right hand barricade portion to make up for the time.