Thanks Marty,
On some rails just changing the flat 2x4 to a 2x6 in the area the cap will be is all thats needed. We just screw it down and where the joints are we take a flat Simpson plate (don't know official name) and run screws from underneath to keep joints even or separating.
With Fortress we order the flange attachment and mioubnt a 2x6 to it and then the cap. With aluminum I order it with a 3" flange welded on.
We let it float over the deck and the outer edsge just lines up with any other caps you're using.
Bobby