[elhigh]

If you don't need to seal the gap between the cap and cab, you might want to consider not doing it.

I know, aerodynamically an uninterrupted skin is best. But I've been driving my truck for over 25 years now and it's had a cap on for all but maybe a year of that. The front of my cap has no window but a gap filler to close up the space between the cap and cab. Over the years, it has worn right through the paint and primer and there is a thin ring of bare steel visible all around the rear window when I take the cap off.

Just a thought.