I'll give $25 for it before you cut it, cause unless you have the correct tool for that, you are going to F it up.
You can cut them with a router, you have to use a guide and protect the surface of the plexi from the bottom of the tool or it will scratch it badly. wax paper works well.
I can't look through the windshield it never worked for me so I just "peek" over the top and get the best of both worlds. So, I completely understand where you are at by not being able to sit below that level.
IMHO:
On your current windshield, Use an erasable marker to find the correct height of the edge you need, then measure from that mark, the size and then buy the correct one. My logic, you'll still have the full size one and you'll also have the one you would have had to buy after you "didn't get the desired results" on the full size one.