What about a stadium seat, which spreads the weight evenly instead of on legs? Many probably have other drawbacks, but some look promising like the Rio Adventure My Pod Seat which is also intended for canoeing.
I have a kayak with similar strap on seat to the inflatables, and it kills me with back pain due to insufficient back support. I just worry about an i12s giving similar results, where my big tall torso just levers down and crushes the back. I have to wait a week or two between outings now due to this.