If you're running a PMR or other custom-install board, you can use a cutout made from your choice of material (everything from nylon to breadboard to cardboard has been used), secure it to the frame using the hole for the top mounting screw on the stock board, then attach your microswitch to the cutout using your favorite method (brad nails, screws and good old epoxy or JB Weld have all been used). Many ways to skin the proverbial cat. If you've got a PMR board, you might want to consider one of these pre-cut board mounts:
http://www.pbnation.com/showthread.php?t=3837993
