Great responses so far! I especially like the idea of a second magnet on the window to keep them open and still armed, as there is a seconday lock on the window that allows you to keep them open a crack.
Would this magnet be sufficient? It looks like it is thin enough, and 10mm is just a hair bigger than 3/8":
http://www.gaussboys.com/ndfeb-magnets/D1002.html
Would this magnet be sufficient? It looks like it is thin enough, and 10mm is just a hair bigger than 3/8":
http://www.gaussboys.com/ndfeb-magnets/D1002.html