I haven't given up, just spent most of the day with more moving. However, I did redesign the cover as I didn't get the screw hole alignment right and now it has a tighter fit too along with a stronger body (thicker on the sides -- tried thin but it was too weak).
I think it is actually a nice and tight fit so I'm going to skip screwing it to the wall on the inside at first. I'll make the lower cleat in just a couple minutes.
Only hangup has been figuring out how to make cut out patterns and/or louvres in Fusion 360 for the fan vent. I know I can just buy one at an RV store so I might do that although the printer is here and the RV store is somewhere else
. I'm going to try to figure that out -- all part of the learning curve of 3D modeling and I've seldom regretted spending time figuring things out.
I also need to pick up some longer bolts for the assembly...