Sorry Bob, had to run a few minutes ago. I think that was the quickest A/C fix ever. Just had to shoot a little freon in it and it's working a lot better. About 10 degree colder air coming out the vents, and at 105 outside, you need all you can get.
I just measured my Kubota carryall, and I have a five foot bottom and back, rather than the four foot I thought I had. I think a four footer would be about perfect for my Yanmar, which is about the same size as your tractor. I got 10foot treated lumber, four 2x8's, and one 2x10, and cut them in half, and of course for a four foot box you would use 8 foot lumber and cut in half. It took me about two hours to construct. After cutting the lumber, I marked the holes from the carryall to the lumber and drilled holes for the bolts. I used flat washers to keep things from pressing into the wood. It was really quite simple. I used the 2x10 at the top of the back and at the end on the bottom (furthest from the tractor), and 2x8's everywhere else. By doing it this way, the bottom lumber just extends past the carryall frame (so you don't catch your leg on the metal frame and just below the frame at the top.
I'm sure this is not as well explained as it could be, but you'll figure it out just by taking measurements if you decide to build one from lumber.
Good luck and keep us posted.