Any reason the fusible-on-both sides stuff (can't think of the right name) wouldn't work? When I saw 'interfacing" it made me think of the fusible-on-one side stuff used in garment making. The couple of times I've tried to applique and pinning the applique to the fabric I kept stabbing myself on the pin and getting the thread caught. Using the fusible stuff would be the best of both worlds. Great idea!
Susan