You need a switch cap with a rim/ring to prevent the cap fallout out of the enclosure - but it might be hard to get one the exact size and spacing you need.
Something like this
http://www.rapidonline.com/Electronic-Components/Buttons-for-12-x-12mm-tactile-switches-71594
or
http://www.rapidonline.com/Electronic-Components/1D-Series-caps-for-tactile-switches-80567
Just sits on top of the switch, if you need to extend it (as I have in the past) you just need a bit of plastic tube (or wood, or anything the right size and shape) - to glue to the underside of the cap to give you the extension you need.
The glued on extender has an advantage too, you can file it down once you screw the case on and discover that one of the switches is permanently held down because you screwed it a bit too tight.