Those of you with toppers, caps, etc. on your pickups...

I get a few dribbles around the Leer Cap but not much.

But I've also put foam tape all around the tailgate opening (sides and bottom) and under the plastic tops of the side walls to seal against dust. There's also a few seams and holes in the bed that can be plugged. Think that helps.

The Leer Cap itself seems pretty tight. Its door has a big lip over the tailgate, and door's sides and top are sealed up when it's clamped shut.

EDIT: and the cap has foam tape under it where it rests on the sides and front of the bed.