plexiglass may be the wrong word for it. I use the plastic sheeting from lowes (lowes home improvement).

it is near the glass cutting section. I think it is about 1/8 inch thick. what I did is buy a sheet about the size of the opening, use a dremel to cut the shape out for the opening, use bolts with fender washers and wing nuts to attach it. the bolts with fender washers is so that it is easily removable if you find you have too much blockage.

I have actually cut the plexi (or similar) with a dremel (cut off wheel) and with a jig saw. if you use a jig saw (or similar) use a metal cutting bit (for the small teeth) and go really slow....really really slow. mine took some time to cut out because of it's size but if you go too fast, it will take out a chunck.

If I can find the pics of my first grill block, I will post it. it may give a better explination to what I did.


******edit*****

this took some digging but here is my first one. notice that I did it in sections so I could take a section off if needed. I also used the fender washers (the large washers) so that there are no extra holes in the car and you can see where I used the jig saw and went too fast. that is where the sharks tooth looking pieces are broke out of the block.

my front end looks completely different now.

https://i49.tinypic.com/dpijpi.jpg