ok. i see what you mean. i usually don't spray such small pieces. but you could cut your backing squares, batting squares and piece the top squares. Then spray the batting square (or the wrong side of the backing square, whichever you prefer) and position them together, then spray the remaining side of the batting square and put your pieced square on top. It will hold it. (sometimes i put a pin or clip on the edges just so nothing gets caught on the feed dogs or presser foot when I am sliding into place on the machine).
And since you are spraying on a relatively small area, I think I would put something underneath, such as a piece of newspaper, plastic tablecloth, or even a sheet which can be thrown into the washer easily. That way you won't have as much trouble with "overspray". That is the only problem about spraying small pieces, as it only takes a light mist to make the fabric or batting sticky, and you are probably going to waste some of your spray.
Hope this helps! I'm off to clean. :cry: I'd much rather be sewing!