" An additional step might be to place a very thin plastic like "Glad cling wrap or equivalent" or other no stick cover on the bottom half to have the silicone form a seal without binding."
A smear of oil or grease on one side only will also do the trick with no risk of wrinkles ...... done that lots when making gaskets for the hull to fin joint on custom wood / fibreglass fins for my windsurfer ... Cheers Martin
