I've heard of people using 3M VHB (very high bond) tape. Two requirements: the area you're installing it in must be clean of wax, oxidation, etc., and you need a very consistent gap between the awning
channel and the roof. Even a 2mm gap can collect dirt and then permit wind/rain/etc to peel it back a little, gradually opening the gap.
VHB tape is what's used on modern car emblems and trim pieces. If the fit is precise, it's "permanent".
For what it's worth, I used the belt and suspenders method to install my rail: drilled holes were filled with silicone caulk, then the butyl tape was applied to the entire rail, and when my screws and/or rivets were attached, the butyl tape "squished" a bit. After all was said and done, I used a Popsicle stick to smooth/push the slightly squirted butyl "putty" back into the gap between the awning
rail and camper.
Note: I did NOT squeeze all of the putty out as I tightened. For the stainless screws, I used nylock nuts to prevent them from backing out.
After the rail was installed, I caulked the "up" side of the rail, then put caulk over the screw heads before installing the vinyl trim that covers the screws.
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