I made one with lexan, lighter than plexiglass but more expensive. I cut th lexan to fit the screen portion then used the brackets that mount mirrors on walls to secure. Worked great :approve:
???? Are you planning on leaving the outside door open? If you are only trying to save some heat I would speculate you are wasting your time and money. I doubt the increased R value will be noticeable.
DW also like to keep the door open, so I took my screen door off, and took it to a local glass shop. Had them remove the screen and replace it with smoke colored plexiglass. They did and excellent job and it works great.