I've got a little insulation in the walls and ceiling, and heavy foam board under the floor. My "hot" temperatures are 85-100, and only for a few weeks at most. I just want come cool air blowing around inside while I take my afternoon nap, tie some flies, or make some lunch. I never need it once the sun goes down. From reading posts, I was under the impression that there was less icing without a return duct, so I mis-read that one. Humidity isn't much of a factor in Idaho, Wyoming, and Montana where I camp. Very dry for the most part. If it gets any humidity at all, then it rains and cools things off pretty quickly. If I can figure out a simple way to start, I might give it a try, and then I can always add the return if I think I need it, as Shadow said. I don't want to give up the space for a permanent install, especially for the short window of need. Even some of the small single and dual hose portables take more space than I'd like to give. Hacking a window unit seems like the best solution if I can get it hacked to start with. Thanks for the feedback.
Brings up a question on the return. I see them all close to the supply. Would it not be more efficient to have the supply high and the return low, or the supply on one wall, and the return on another? To promote air flow in the room rather than just a quick in and out in one small area? Maybe I'm overthinking it, or don't have a clue.

Living in the arctic hasn't taught me much about A/C over the years. With heat, I want my supply coming in low so it rises and helps lower areas stay warmer. I ran the BTU calculator, and I'm right in the ballpark, needing between 4500 and 5600 btu depending on the interpretation of "climate" and "insulation." Average for both was right around 5,200.