I've used truck caps of various sizes over the years, some large enough to have a pass-thru door in back and about 5' inside height (but just a cap, not a camper), and some just same height as the cab, and never had any problems with any of them shifting over rough roads, back country camping, Titus Canyon in Death Valley, or at highway speeds for hours on end.
All had weatherstripping of one sort or another run 'round the edge of the pickup bed, tight at the corners. Adhesive on one side. I remember one weatherstripping I liked was gray, about 1.25" - 1.5" wide, was super adhesive with a paper release and easy to align on the truck, and easy to get off when it came time to sell the cap.
Most of the caps I've done were just C-clamped down, and tight, checked often, though one cap was bolted through with fender washers to each side.