I've never done a plexi cab window, but if it's like the glass and fits in with the gasket they are a pain.
You've got to make sure the gasket is clean. Try and match it up with where it was on the frame, exactly. Make sure the keyway is unlocked and pull the cord out.
I use electricians tape, one wrap around the gasket and frame to keep the gasket in place. I use silicon spray like wd-40 as a lube. Have a razor blade handy, and you need some picks or awls to help get the gasket around the glass.
You need to have it on a flat surface like a table that you can walk around and/or spin the whole thing to work on it.
Start at one corner and work the glass into the gasket. Slit the tape with the blade and remove the tape as you work around. Keep it lubed and work your way around the frame with the picks. If it is a glass window, don't pry to hard against the glass with the picks. The first third of the way around is critical, when the glass may want to pop out.
After you get aobut two thirds of the way around, it starts coming together. You might have to bumpt it with hands on either side of the glass, the whole pane is above the framne when it's done.
Install the cord on the inside, then go to the front side and tuck the lock.
It takes me about an hour and a lot of cussing to do this. I'd like to see how the pro's do it......=).