I know this might sound a bit rough but it actually looks ok and works really well on the darker tinted rear sliding windows of the T5 Kombi Beach.

Make a square out of two inch Water Proof Black Gaffa Tape with inside dimensions of the square the same size as the window opening.

Lay down a square of fly screen material with outside dimensions one inch smaller than the outside of the gaffa tape square.

With four hands stick the pre-prepared sticky fly-screen squares to cover the sliding window opening on the outside of the van.

Mine have been on for 7 months so far and comments are that it still looks good (1mm of the white sticky is now visible at the edges). You can drive with the windows open and the fly-screen stays in place. My kids have pushed on it from the inside and it doesn't seem to have moved. Provides complete freedom of operation of the sliding windows. Amazing stuff that gaffa tape.