Up until this spring, I used a simple cotton drop cloth, cut to size and with a loop sewn into one edge, hanging on a piece of conduit that hangs above the front window. This was mostly for privacy, but as the days got longer and the temps crept up, I started thinking about how to mitigate heat gain. The bad thing about my curtain, and also poorly-fitting sunshades inside a car windshield, is that the gap between the window and the shade can result in a mini-greenhouse effect. The air between the window and the shade gets heated, rises, and (relatively) cooler air is pulled in from the bottom. The same effect can happen with cold temps, except the air is cooled, sinks, and pulls in warmer air from the top to be cooled. It's actually a very good way to passively heat a home using south-facing windows, if that's what you want.
So I cut some rectangles out of Reflectix and have them wedged flat against the front glass. It lacks the esthetics & portability of curtains that are easily opened/closed/stowed but they seemed to be doing the job. Full disclosure: I moved into a house for the rest of the summer. For reasons I won't bore you with, I didn't want to get into modifying the bus for Tucson's fairly brutal summer and couldn't relocate to a cooler locale.
