I have 2 things you can try. Either poke some holes in the middle so the foil doesn't get sucked down into the bowl, or you can use these clips. http://www2.grynx.com/wp/projects/Bi...s/IMG_5623.JPG
I use them for my small funnel bowl and they work well. You just take off the silver pieces and put then into the hole in the funnel. Then bend them into place. They will hold your foil up and keep it from sagging.