Windows at my place do not have this nice feature of remaining locked half open. I have therefore designed this particular item which does the trick. Printing a bunch of these was way cheaper than having all the windows replaced (and faster than waiting for my landlord to approve the replacement). 

Designed with freecad: the source file is available for you to download.

PS: clearances are thight on this one. It prints like a charm on my mini, but my ender3 had a hard time with it.