I had been trying to clamp cheap 3/32" widow glass to the bed. The clamps were in the way, and the glass bowed. I came up with this: float a piece of glass on the bed with high temp silicon glue from Harbor Freight, this gives a very flat surface. Now take another piece of glass and clamp too the glass beneath. The key is to make the glass that is glued to the bed larger than the bed. I take the doors of my R2's and you may need to as well. You now have a really flat easily swapable surface. Update: On my second bed I covered the bed with the silicon RTV before placing the glass on in. I guess the RTV shrinks as it cures and was able to distort the 3/32" thick glass plate. I guess I need thicker glass or I should go back to less glue.