I took a different approach. Didn't want any extra openings in the trailer wall. I saw a post by someone here named WA a year or two ago who built a nice little conversion with a permanent half box in the back to hold a window unit.
I really liked his idea, but we needed to be able to remove the AC box when the weather was cooler. The fantastic fan with our window configuration cools in all but the hottest weather. We also needed to be able to unload kayaks and bikes as well.
Here is my solution: A removable, insulated box to hold an inexpensive window unit. The box is primarily supported by the now infamous vertical e-track and a couple of 2 x 4s.
Here's the box before finishing. Insulated with 1" styrofoam, and sealed mating surfaces.

The box slides into the e-track fittings, then is secured by carriage bolts and wingnuts. Also attached cargo door side wall at top and bottom with brackets.

When using the A/C, I open the left door to exhaust the hot air, and close the right door to seal the trailer interior. I put a simple brace to hold the door in the slight open position, and installed a CPVC condensate drain with a removable pipe to route the water out to the ground. (as far as the open door...I'll decide what to do about a thunderstorm when it occurs)

Here is the door arrangement in the"unloading mode".

Here's the view from the inside. This is a 6,500 BTU Frigidaire, it is much quieter than I thought and cools the trailer down pretty fast. I have the trailer insulated with 1" styrofoam throughout and fiberglass in the upper curve sections. It really makes a difference.

It would have been easier to install a roof unit of course, but it would have cost me a little more, and wouldn't have been as much fun.
Thanks again to all on this forum who are inspiring and willing to share ideas. See y'all on the rivers or the mountain bike trails...
(And thanks "WA")
David