My trailer originally had a air conditioner that had a metal bottom and was designed for flat roofing. The span from the rib in the ceiling to the next aft rib was 40", the front rib is right at the front of the 14" cutout. This was not an ideal statement by any stretch. The front of the air conditioner had support, everything aft was sitting on skin not supported by anything. The weight of the air conditioner pushed down on the skin as expected and one rear corner actually wore a crack in the skin. So I needed to repair that first.
Photo of the crack before repair:
Photo of the patched crack, I drilled holes at the ends of the cracks to stop spreading, drill just slipped through, not much left at the spots, the spacing was uneven on one end to clear the longitudinal support in the ceiling;
Now I needed to add some support for the future air conditioner under that 40" span. I used some rigid foam insulation and used a compass to scribe the arch from the bottom of a rib onto the insulation piece and then cut that with a razor knife and fine tuned to get the curve pretty close. I then used that to trace the shape onto a piece of 1/8" thick 2" aluminum bar and cut that out creating a web so it would match the arch. This took about 6 jig saw blades to make 2 of the support brackets. I then used 1/8" x 1.5" and cut to length to fit in the ceiling to be an flat arch flange to place on top of the web arch. Don't want a sharp 1/8" edge pushing up on the skin wearing through it, plus the 1.5" web smooth's out the curve a bit. I pit a couple pieces on the ends to screw sideways into the longitudinal supports in the ceiling.
This is what the bracket looks like once it was welded together:
This is what the brackets will look like once installed. Still need to put screws through the side flanges to support them. I'm planning on putting some polyurethane sealant between the support and the skin to minimize possible friction and wear. Not sure if I'm going to rivet them through the ceiling with some Olympic rivets or not, they'd be under the AC so shouldn't leak if I do. I put my body weight on them and they don't flex, so this should support a new AC. Now I just need to decide which one I want.
Difficulty level, 2/5 hammers, cutting the metal 4/5 hammers because it chewed up the blades and was slow.