I cannot address the double panel situation but as for the headliner I see no reason why you could not use the metallic backed rubber roof repair material I used on my 57' BelAir. Being metal backed should allow the same adhesive to be used and it's only about 2.5-3mm thick. I doubt it would even be noticeable under the headliner. It can be purchased from audio shops but I got a very very similar product from Home Depot here in the States. It is in a 6" wide roll and was 1/3 the price of the audio guys. Has an excellent adhesive pre-applied. I think I have a piece left if you'd like a sample.
