The trailing edge is 1-1/2" angle aluminum. The top edge of this is spaced about 3/16" below where the face skin will lay, allowing for a strip of rubber baseboard to lay over the deflector-cap gap. I haven't procured any yet, still hoping to find or salvage a scrap rather than buy a roll to get the needed 42" length.
First trial fit (without fastening) looks pretty good. The center truss is about 1/4" proud of the other two at the leading edge, tapering to dead even at the trailing edge. I'll wait until I attach it to the truck to correct this, as it could change. I'm not sufficiently skilled or equipped to create a compound curve, so I will get it so that a straight edge lays crosswise at all points before making the face. I need to turn my attention to less interesting projects for a couple of days, but I'm excited about this progress and will get back to it soon!