Topper Mounting Problem Caused by Narrow Bedrails

The bedrails on my 1998 Tacoma are only about an inch wide, and I want to mount a topper over the bed. I don't like J-clamps so I'm going to hard-fasten the topper using nuts and screws. To widen the top surface of the bedrail I fashioned some runners using 1-inch aluminum L-stock and was going to screw the runners to the inner edge of the bedrails. Then I would screw the topper to the horizontal surface of the runner.

Unfortunately, the vertical edge of the bedrail is not perpendicular to the rail's top surface. When I attach the right-angle runner to the slightly-out-of-plumb bedrail the top surface of the runner will tilt up towards the center of the bed. Please see the attached crude drawing.



What would be a good way to overcome this problem? Would using rubber mounting gaskets (available by the foot) for the seal be a better solution than using the usual foam tape? Any and all comments are appreciated.

 

The previous owner of my truck used c-clamps and cut notches in the topper (v shaped).

 

Shim it.

 

Aw that's too simple

Seriously, I thought about that but due to the way the base of the topper is molded (it has an inside lip that I forgot to include in my drawing), I thought that placing enough shims to level the runners would bring the edge of the runners too far away from the bedrail to fit the base of the topper.

I'll go out to the garage and remeasure everything - maybe the topper will indeed mount properly. If not, maybe I can install the runner and use vicegrips to slowly and carefully bend the vertical part of the rail so it is 90 degrees from the top surface.

Thanks for the idea!