Eric,
Have you thought about getting an Exped Downmat?
I got one on winter sale last year from MooseJaw for $69. It comes with a double layer stuff sack that doubles as a air pump.

I checked the size of the valve on a ThermaRest and it is smaller than what is on the Exped. So, you couldn't just get a stuff sack to use with your TR.

The Exped Downmat 7 weighs about 34 oz. I really enjoy having it on winter campouts.
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If you think you can, you can. If you think you can't, you can't. Either way, you're right.