If the double sided tape doesn't stick, get some better tape, like the stuff 3M makes for this exact purpose. Then make sure you've cleaned off the fiberglass properly, using a good degreaser. The fiberglass probably either has mold release or residue from being polished after removal from the mold. Fiberglass is really easy for 3M tape to stick to, you just have to make sure it's clean. You're trying to get the tape to stick to the smooth gelcoated top surface, not the rough back, right??