Thanks! I had considered the 20 or 30 mil marine plastic sheeting. I didn't discard it, just went another more readily available direction.
BTW, total costs to date are:
Windows, paint, and screws $152
Miscellaneous hardware ~$20
Cab build total costs $172.
Now, I had the wood, metal, and T111 sitting around. If I had to add those I think the costs would be ~ $75 higher. My cost baseline was the Sears $239 soft cab which would still require a frame or modifications to the one provided.
For those with a GC 23/4/600, attaching on the FEL and ROP mounts makes for a very secure cab. I was somewhat concerned over the strength of the metal I used as it flexed some, but once the cross members were installed the whole thing got very firm. At this time/version the whole cab will come off by removing 10 nuts and bolts and one single screw. The top will come off with removal of four screws.
I just bought a hydraulic motor on ebay for converting from the manual chute rotator, and will soon look for light options. I may eventually need some heat. By the time this whole project is finalized I think it will have cost $300-350 for a hard, heated lights nearly weather proof cab.
The real benefit to DIY is its easy to change/improve after the fact. You built it, you know where/what can be improved.