For warmer in that size constraint you might try the aluminized mylar bivy bags.

For sturdier in that size constraint you might try contractor grade garbage bags.

If you can carry something larger I would suggest getting a sil-poncho from ID. It will only weigh around 10 oz but it will be bulkier than a back pants pocket. Might fit a cargo pocket but it would be noticeable in bulk. I carry a sil-nylon tarp in the back pocket of my vest but that is a full game pocket covering the entire back so when I fold the tarp flat it covers my entire back and doesn't form a lump.