I needed to mount our Yakima rack to the fiberglass A.R.E. shell on our Alaska Tundra, and since our Yakima towers have standard rain-gutter clamps, I needed artificial rain gutters. After some searching, I decided on a set from the oddly named Lkonwee on Amazon. They weren't cheap at $52, but after installing them, I'm convinced I actually got an excellent buy.
The brackets can be mounted vertically on the side of the shell, or horizontally on the roof. I chose side mounting to keep the overall height of truck and boats as low as possible.
The brackets came with full-size backing plates for strength, and were nicely powder-coated, with matching black-anodized hardware. The square-shouldered carriage bolts fit through square holes in the brackets, meaning installation is a one-person job since you don't need anyone on the the outside holding a wrench on the head of the bolt. It also makes for a much cleaner appearance.
The gaskets, too, were excellent, with a double ring around the bolt holes and a nice raised perimeter.
The gutter itself is deep and secure, and two small raised bits on the bottom would help prevent the rack sliding if you got hit by a big gust of wind or had to panic brake. Well-recommended.
One installation note: Our shell is lined with an ozite-type material, which wasn't completely glued down where I drilled the first hole, with the result that it bunched up and twisted around the drill bit, leaving a fat wrinkle no matter what I did to push it back into shape. For the remainder of the holes I was careful to barely drill through the fiberglass, then poke a hole in the carpet with an awl.
Lkonwee brackets are on Amazon here.