Window screen plus magnets plus Jetta.
The magnets are in aisle 10 at Canadian Tire in Charlottetown, where the nuts and bolts are displayed; personally I would have put them with the Velcro or the glue, but I can see their logic just as well.
Postscript: it worked like a charm. Not only were we cool (and cooler as the night wore on), but we didn’t have to run the fan from time to time to defog the windshield. It felt weird being at the drive-in with the windows “open,” as we’ve always had to keep them closed to keep the bugs out. The cart next to us was a VW Beetle convertible that kept its top down throughout the film, and dealt with the bugs via bug spray; this mean we also had to be quiet(er).