I do a lot of kits in multiples, so I have cut out foam masks for each kit to fit into the wheel wells.
You just spray the wheel wells, pop in the foam masks with tweezers, they expand out to the well edges, and you spray the underside color,,,,,,,picking the masks out and saving them for next time.
You can get the foam in small sheets at Wal-Mart and Hobby Lobby, they are 5 by 7 inches or so, and come in a stack of 50 or more different colored sheets of "Sticky backed foam". I used to peel off the backing and stick them in place,,,,,,,but, I have found they work just as well leaving the backing on.
Rex