I recently completed my Cub Carrier. A big thanks to Kodiak and SgtBull and all you others for some guidance and basic dimensions to get me started.
Frame is mainly 2x2x1/4 angle from some drops picked up for free from work. I went with slightly smaller boards. 5/4 decking for the floor boards and 1x for the vertical boards. I was trying to keep the carrier tight to the Cub in order to maximize lift capacity. It is behind the rear tires, but if for some reason I ever have to move the tires in, it might take a couple of passes with the saw to narrow the boards. I used a two bolt variation of SgtBull's brackets on the drawbar to mount it.
And with last night's temps dropping into the 30s, I put the carrier to immediate use with some firewood. The Cub lifted and toted it without a hiccup. I plan on making a rear board and some removable sides next.