You're right that that G/flex is great stuff. I've used it successfully on ABS and polyethylene. This link
is the best I've found.
Once the plastic to G/Flex adhesion is established, standard West Systems can be used -- at a lower cost and ease of use.
I was able to get the endcap on mine out from underneath the ceiling panels by drilling about 2-3 feet of rivets in the ceiling. That way, the aluminum sheet could bend and not crease. Lower walls were off at the time. Hidden rivets did cause much swearing and encouraged an additional adult beverage or two.