the picture frame glass from pound/buck stores is 2mm thick and plenty strong enough. easiest way is to buy a4 picture frames and cut the glass down to size.
As far as the z switch thing goes. An extra 2mm doesn't effect the limit switch as it's calibrated slightly below it anyway. Or I'm thinking of the wrong thing.
And little corner brackets should work so the glass just sits in the bracket for easy removal. The problem with clips is that the print head clips them if you print something big.