Originally it had a 1/8 " piece of Plexiglas, but that broke while test fitting it after I added the plastic corner molding.
So then I used 1/4 " Plexiglas ( might have been Lexan) and cut it using a jigsaw. I used the existing hole in the door as a template.
It's glued in place with the same 3M 5200 I used for the other windows.
If you look in the original build thread, you'll see Mike used a router and a circle cutting guide to cut the foam and also made a larger circle on the inside.
That left a place for the Plexiglas to be recessed into. You can see that here on Mikes thread... (I don't know how to post somebody elses pictures)
viewtopic.php?f=55&t=44091&start=525 Page 2 of my thread (Big Pink) has some pictures of when I skinned the door.

Page 10 shows the Plexiglas difference, but it's kinda hard to see.

I like the Wiley windows, but didn't see a need to have a functional window on the door, since I have other windows.