Just an idea on forming the patch. Is it possible to fabricate a shape out of something close to what the patch has to cover then using a heat gun (electric paint stripper or a hair dryer on steroids) almost melt the patch material so it will take a set in the desired shape? Then use Boxer-100 adhesive to do the patch. This is a slow acting solvent so it won't aggressively attack either the liner or the patch but it does take a while to set. Like days for a good patch maybe. You can hold the patch in place with duct tape.
As I said...just a thought.
Al