We are still working on our bug bus, but my wife made special inserts out of 1/8 inch think poster board cut out to fit inside the window openings, placed heavy clear tape ofer the eges leaving tabs so that one can grip them to get them out of the window, and we use a sunscreen for the windshield of our Shortie.
That was a lot of work, but it made my wife happy to contribute to the cause, and it really works well for privacy and insulation.
I like the idea of using a bungy across the vehicle right past the driver's seat to hang a curtain from. That doesn;t take care of the other windows, but it would be quick and easy for the front. Thanks for the tip!