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i've used 1/4" (1/8"doubled) under my legs in the 20's. several times. i wore down booties though and had stuffsac under the knees

i think you're better off with ccf, ccf that small won't take up much space and should be alot less than 8oz. maybe around 3oz. may not work as good in the thermarest chair though. speaking of, what...

i'm thinking the new neo air may come in a super short version, but i could be wrong

another option is to put some spare clothes/rain jacket ect in a stuffsac and use that under the knees and a short pad for the feet.