The one mentioned is plastic. Here's a pic of it and more of their selection. I am not in the market (Have an extra plastic window top), but was curios.
I went with the original plastic. I understand the benefits of the glass retrofit but it's never too cold here in SoCal to operate the top. Also, the field of vision is drastically reduced and it's bad enough already. I can keep the plastic clear with care.
I only wish they had a zipper window option. That would be way cool to drive with the windows (all 3) down and the top up for the shade (read skin cancer which is a problem for us old farts that have spent a lifetime outdoors).