We did this a month ago. Here's a link:
http://s513.photobucket.com/albums/t332/kathyanddave88
We got the plexi at Laird Signs in Iron Bridge, Ontario, north of Lake Superior. They cut the plexi to fit on the spot. A family business who took great care of us when we stopped on a whim.
The gap between the screen and the inside edge of the screen frame is 5/16". The plexi is 1/8" thick. The remaining 3/16" is made up by the fittings you see, which are from Lowes. They have knurled knobs that can be loosened and tightened by hand without falling off the door. This is all described in another recent thread.