I have been using an old style Therm-a-rest inflatable pad I picked up for half price because of a blemish. I love it and has served me well, but REI does not carry the new model. There is an REI branded version https://www.rei.com/product/179042/rei-co-op-sit-pad If I did not not already have it, I would either use the Therm-a-Rest Z-lite Seat Pad that @REI-JohnJ mentioned or cut one from an old or cheap closed cell foam pad. The new Therm-a-rest  and similar and lower priced items, are available from that big online retailer that begins with "A".  Amazon.com : Therm-a-Rest Lite Seat Ultralight Inflatable Seat Cushion, Blue, 13 x 16 Inches : Sport...

aka "Boonerelli"

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