I'm planning a rack for solar panels on the roof of a cargo trailer. I will mount the panels to a pair of 15 series 80/20 aluminum extrusion rails. I will mount the rails to the roof of the trailer using aluminum angles. Here's a view from a Sketchup model giving a rough idea:
My question revolves around mounting the set of aluminum angles to the trailer's steel roof beams. Here is a rough model showing the relevant parts:
The aluminum angle is 1/4" 6061-T6 with 2" legs cut to 2" wide. The steel beam in the trailer roof is 1" wide, 1-1/4" tall, and about 1/8" thick. Between the angle and the beam is the thin aluminum skin of the trailer.
The interior of the trailer is finished and prevents any access to the beams from the interior so I can't use through-bolts to attach the angle to the beam.
So my current plan is to drill and tap, from the outside, the top of the beam. Then I can attach the angle to the beam using two bolts that screw into the top of the beam. I plan to use 1/4"-28 bolts. I also plan to use VHB tape between the aluminum angle and the aluminum roof skin.
Finally, the actual question. Since the steel beam is only 1/8" thick, is the choice of using 1/4"-28 bolts strong enough? Or would 1/4"-20 be better? My thought is finer threads gives me a better grip over courser threads.
Side question (that may be off-topic) - is there a better, stronger way to attach the aluminum angle to the steel roof beams than screwing in bolts given that the steel is only 1/8" thick but I can't use through-bolts?