I wouldn't even entertain the adhesive idea.

1. These racks are exposed to constant dynamic loading....bounce, bounce, etc. and would really test your adhesive of choice.
Mechanical fasteners rule for this app.

2. The new, deluxe canopies are very expensive and who wants to go hacking into that nice headliner that you paid dearly for, just to get to a gluing surface that may or may not work.