Yes it is a real problem. Have been lucky myself, but mate did a rear window on the Birdsville track last trip.
Have always just used cardboard taped on but now that we are doing more trips and I have more time, made up one yesterday.
Went to a tyre dealer and got an old corflute sign, cut to size, then fastened on with suction caps from Bunnings. Got the ones with a hook on them, so that the cup end fits in a 9mm hole and the little hook fits in to hold it on. Can easily be removed by taking out the hooks. Simple, cheap and I hope effective.