Don't drill...add a snap
Don't drill if you don't have to. The camper shell for my 23.5 didn't fit the snap pattern either (I bought mine over the internet and the boat already had the opposing snaps installed). After much study...DING!!! The light went off. Just add another snap to the shell. Instead of worrying about drilling, filling, leaks and all that fun stuff. Go to West Marine and get a set of snaps (maybe $5) and just add another snap where you need it.Ken