I'm not planning on a full cabinet but just a shelf. Paper towel holder, lantern, misc little stuff. Probably 38" wide, 10" or so down from the roof, and maybe 8" deep.I'm asking questions because what you wrote and the responses had me puzzled.
What is the depth and distance from roof to shelf you are looking for? is it a shelf for light or heavy stuff?
I'm revisiting the shelf idea because I blocked 50% of the passenger slider and will put a 6-10 deep shelf about 3 inches down from the roof, for light stuff. I'll mount wooden blocks to the angle metal and to the forward side of the existing upper wall cabinet. Then glue and screw the shelf up from the bottom into the wooden blocks. Put a lip on the shelf, nothing attached to the roof. It will be crude but it will work.
Didn't catch the build link in your signature before.