I've done that with a gallon ziplock bag, maybe a third full of air.
If held the air, but keprt slipping out from under - I gave up on that idea.

 I just use a stuffbag with clothes, or more oftan, just my fleece jacket folded up into a 4 or 5 inch thick pile, with that on top of my spread out PFD.  The pfd is propped up/leaning against the tent well, and that helps keep the bag from contacting the tent wall as well as adding to my "pillow"