Antenna bolt keeps backing out...

I have a Firestik "non-ground plane" cb antenna on my fiberglass topper/camper shell. It's mounted on a medium duty spring, the bolt goes through the fiberglass then screws into the spring. My question is, the fiberglass was to thick to use the lock washer that comes with the kit, so can I use thred locker to keep the bolt from backing out without affecting the signal? Or...should I get a little longer bolt?

 

Get a longer bolt and use lock-tite on it.