I don't usually use anything under my quilt when spraying because I try not to get any on my counter. I use a large piece of cardboard to hold/place on the sides to catch any over spray, also point the spray toward the inside of the quilt. After spraying the quilt, for any residue I spray 409 on the counter, let it set for one minute and wipe clean. If you really want something under the quilt, you can buy a thin piece of plastic 10 foot by 10 foot at the Dollar Tree for 1.00. Love the Dollar Tree and Harbor Freight for sewing items. Speaking of Harbor Freight, I bought the small magnetic bowls for my pins, bobbins etc for 2.99.*** at JoAnn's they cost 9.99.
From the dollar store, I use their washable glue sticks, the thin plastic for a lot of things, their heavy duty zip up bags for storing my quilts in once they are done, their large clips for holding my quilts to my counter, their containers for my scraps, zippers etc.