The general concenus on comfor is a carved seat from a block of closed-cell foam.  A back band is usually all that is needed because if:  You sit up straight rather than lounge, one really doesn't need much of a "backrest".  The Pax 20 is a sport/fitness craft, not a all day cruiser.  Another source of pain can be how the legs are positioned.  Knees bent and more together than apart can, in some cases, keeps the butt and or legs from hurting or going into some stage of numbness.   Just my expierence.  Hope it helps.

I'm considering using the Pax 20 hull as a platform for an ocean Racing ski because this boat is built for racing/training. As mentioned, it is not really designed for a laid back paddling style.

You might like to consider a product like the one used in this craft if you don't want to carve your own. Its what I will probaly fit my pax 20/ocean ski with http://www.surfski.info/content/view/650/147/

The "bumfortable" is from NZ but I'm sure there would be something similar available in the states

i had a pax for a while and its truly a RACING boat it wasnt meant to be comforatable i sold it and built the shearwater 17 now im as comfy as can be

I used a Harmony backband anchored to the side of the hull. It tended to roll backwards when I sat in the boat due to lack of support from the low aft deck. I corrected this by placing a small block of foam between the deck and backband to prevent this. I used a simple air cushion as a seat (Sealline I think). I've used this boat for 7 years and this has worked very well.