Glue-on velcro screens:
See here:
viewtopic.php?f=41&t=16038He mentions where you can buy them in a response.
I make my own myself, you can buy nylon screen for a couple of dollars a metre in the hardware shops on Main St. I just smear some contact cement ($20 a tin) and press it down to the aluminium window frame; leaving one window in a set unscreened so I can open it and push the other windows open or closed.
Or you can use double-sided tape, a bit less messy.
That lasts for years.
When you need to remove or replace them, just pull them off and rub off the glue with some turps.