When I lived out in WA, I did some searching around town and found a plexiglass shop. I didn't have mine cut down, but I did have him copy it and make me a new one in clear. (came out looking great!) I bet if you were to find a shop like that in your area that they would be able to do it.
Another option, and I've never cut one before, but I'd think that if you took some masking tape and laid out where you wanted it cut, you could cut it then use a couple files and get it brought down to match your tape line. That should work to make a straight cut I'd think? I'll be trying that one day on my original smoked one that's hard to see through. I think I'll need a shorter one for some more airflow in the summer months here. Good luck and let us know what you end up doing.
Brian