I think I am using 1/4" but I will have to measure to be sure - could be 3/16". I would also stay away from 1/8" for the same reasons.

I bought from that Mcmaster I think and it is not exactly the same size as the build plate. Also got some off eBay and it is not the same size either, it's slightly narrow side to side and front to back it overhangs the plate. I think the QIDI build plate is slightly different size because the glass fits perfect on the FlashForge creator Pro build plate. Not a big deal though.