I'm with Sparky on using liquid dish soap.
A liberal application, and get a couple of plastic putty knives to compress and push the sides down under the retaining edges of the metal cover. It would help to have a work surface with a V to support and rest the metal cover into, probably laying on top of an old towel (don't use the Mrs's good towels -
) while you push those edges down and into the channel.
Let us know how things turn out.
