For extended sitting while hunting I carry a 1/8" Gossamer Gear foam pad stuffed into a Thermarest compack chair kit. The foam pad isn't exactly the correct dimensions but it functions.

I use the pad on the ground at night to help protect my exped.

http://gossamergear.com/sleeping.html

http://www.cascadedesigns.com/therm-a-rest/seating/chair-kits/compack-chair-kit/product

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