I actually use 0.03" PEI (Ultem) stuck to cheap mirror glass using 3M 486MP double sided tape. One of my kids had a 12x12 decorative mirrors in their room that they decided to get rid of. I got rid of the frame and then used a glass cutter to cut it down to the size of the bed. I used a metal straight edge and a glass cutter to score the glass and flipped the glass over on a couple of silicon baking mats (has some give) and pushed down on the score and it broke nicely. A lot of people use borosilicate glass because of the heat, but since it heats up and cools down fairly slowly and evenly I haven't had it break due to thermal shock. It's fairly thin though so it'd probably be easy to break by driving the nozzle into it.

I printed a shim for the back to hit the endstop. I used a different one, but if I was doing it over, I'd probably print this one so it'd stay in place better: http://www.thingiverse.com/thing:515065

I don't clip mine with binder clips because I don't wanna fuss too much avoiding having the nozzle hit them, so I printed two corners that "frame" it sorta, but what holds it laterally is "gino" pads that are for fastening thermal heatsinks, but work great for this. I bought a 12x12 sheet from Amazon and cut it to the size I needed. Most folks cut it into say 2" squares and distribute around the bed with space between.