You could use translucent plexiglass; lots cheaper and easier to work than Lexan.
I think trying to attach it to the glass would be ugly no matter how you did it as you'd have to look at the tape/adhesive and all the junk that leaks
behind it through the real window. Not familiar with the Scamp
guard hinge setup, so no help there.
You might consider cutting an opening in the gravel shield flat area and attaching a "window" of translucent plastic to let a bit of light
in. Could always add a Scamp
decal (or something creative) on top of that.
If you didn't have to open/close the guard (except for cleaning the window) it wouldn't be much of a problem any more.