http://www.toyota-4runner.org/3rd-gen-t4rs/174536-my-4-dollar-window-screen.html
Not advisable to drive around with this setup, but it's cheap and works well for camping/sleeping in the back of the rig.
Just buy one of these and cut to size with some sharp scissors.
Amerimax Home Products Snap-In Filter Brown Gutter Guard-86379 - The Home Depot
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Agreed, this works great! Did it for a trip last year. Looked at a few other solutions online but this was the cheapest and easiest and they hold in place surprisingly well. Also cut one for the back window (lift gate) although it was a little more tricky because of the curve and width (sorry no pics). For the lift gate I needed one full length piece and then a little 6" or so piece and round off the ends to fit the contours of the window and frame. The pieces store easily too and I ended up keeping them in the Runner for travelling in case we had to pull over and catch a quick snooze. They take two seconds to install.