I've happily used heavy duty velcro or even 'industrial strength' to secure things like flashlights in odd places. Since I live in 80% humidity most of the year, the adhesive doesn't last that long. It seems important to both clean the surface (alcohol wipes work well) and roughen it (light sandpaper) before attaching the sticky stuff.
BTW (and relevant to your other thread
), I mounted my awning rafter (in 2 pieces) and pole to a skinny spot in the back with velcro.