Here's a simple, inexpensive way of adding a bit more security to your sliding windows. All I've used are items readily available from most camping shops, namely 19mm and 16mm aluminium tube, 16mm plastic c-clamp, walking stick rubber and a thumb screw clamp. If you don't need any adjustment, you can do away with the thumb screw clamp and the 16mm tube and just cut the 19mm tube to length.