I built spool adapters for a Yanmar YM2000. It wasn't very complex at all: Some large diameter tubing, a couple piece on each end cut to match the bolt patterns, and bolt it all together. It made the machine obnoxiously wide, and the tires were not exactly the same size; I used what I had: The tractor's original 9.5-24 and a pair of 8.3-24 tires on the outside. They worked pretty well, I guess. It definitely improved traction and floatation, but I have since taken them off and substituted 180 lb wheel weights.
On my machine, the tractor became too wide for its wheelbase, and was very twitchy and difficult to drive in a straight line. A stupidly long description about what I did and why is here:
http://www.tractorbynet.com/forums/yanmar/195084-frivolously-productive-afternoon.html I used Schedule 80 pipe for the spools with 1/4" plate for the ends; if I were doing one for a larger tractor I might step up to 3/8" or 1/2", but I figured the end plates are rather thicker than the wheel centers, so I don't think they'll bend or break first.
That Yanmar was designed to be fitted with duals from the factory, so I wasn't worried about breaking anything. Overall, I can't really recommend doing it, but it definitely improves the traction and floatation. I just hated how it made the tractor handle, and took them off soon thereafter.
