Be sure to check the clearance at full turn left and right while the unit is in full oscillation and articulation. My arm rests rub on the engine cover at the end of those travels. Therefore, if I would add a cab that enclosed my armrests, even if they touched the armrests, it would be making contact with the engine cover. By the way, that is with the seat fully back.
My thoughts on a cab would be to enclose the front, first, with something solid like sheet metal or a fiberglass panel and then cut out a window. What would be a good, solid window material: plexiglass or lexan? For the sides and rear, I would be happy with soft curtains. They sell some pretty tough tarps that could be used for the fabric and the vinyl for the windows is available at fabric stores. Really, the only time I get damp is if there is a heavy, wet snow that falls past the ROPS and lands on my thighs. Even then, I am usually done with the drive so fast that it doesn't have time to soak my coveralls.
For ventillation there would have to be some kind of fan to keep the fog off the windows. The Giant cooling fan on top of the hydraulic cooler comes to mind. Maybe some sort of fabric sleeve could be directed towards the rear of the cab. The only problem I see with this is the potential to gas your self with exhaust fumes. /forums/images/graemlins/shocked.gif Maybe a small, electric fan directed at the front window would do a safer job. /forums/images/graemlins/tongue.gif
I still swear I saw a hard cab on a 400 series unit before I purchased ours in 2001. Gosh, I wish I would have saved that. /forums/images/graemlins/mad.gif