I wonder if you guys could modify your beds to use what Makerfarm uses and sells? It does not warp at all and you just put an inexpensive piece of glass over it that you can get at the hardware store for a couple of dollars. The glass clamps down with small black spring clips.

http://www.makerfarm.com/index.php/p...-heat-bed.html

Here is a photo of one corner of mine:

MakerfarmBed.jpg