I have not used that company however a yellow pages search in your area might turn up a local place that will custom cut your plex for you. Look under plastics or rubber supplies. That way you avoid shipping prices and issues with cutting the plex. 1/8" thick is enough.
Remember plex and even lexan will scratch and get duller over time.
FYI, I used smoked plex on my cab with a CRT monitor and did not like it at all. Made everything too dark and muted the colors too much for me. Made me wish I had gone with clear plex or glass.
Good Luck!
ARCADIAC!