Shell/Cap Owners: How to secure to rails?

I'm looking into a Snug Top Super Sport. They clamp onto the composite cargo rails. I've read at least one review where the cap slid about an inch after some wheelin'. Well, since I do wheel the truck, is there a better more secure way to install these? I'd hate to have the thing slid right off.

 

I have an ARE that attaches to the bed rails. It's been on for 2 years and hasn't moved a bit. Mine has rubber on the bottom of the cap where it sits on the bed rail. That compresses down when you attach the shell to the bed rails and must be tacky enough to keep it in place.

 

I have an ARE and the fiberglass overlaps the rails and bolts to the rails. It only overlaps where the four mounting points are and the cleats still kinda work. It doesn't move.