I've received a few PM's asking for measurements and pics of my rear tire carrier since I mentioned giving one away (long story). I can't send pics via PM, so I figured might as well start a thread. This way you won't be alone in building it and others can chime in ideas to help.
Mine is an old Smitty, I'm not sure if it's old enough to be American made or after they turned Chinese, but all I can say is it's held up well and I didn't have any install problems that I've read so much about. I didn't need to drop the gas tank, and any rattling problems were solved in 5 minutes with temporary/permanent Jerry Riggs...
Anyway, most questions were concerning the tire carrier part.
The tire carrier is very simple, and I think it would be easy to build. If I were making my own, I would build the bumper itself differently for more clearance. It sticks out a bit from the rear, which makes a nice step for getting things in and out of the back, but I could see where it would drag on obstacles. I think the hardest part will be attaching the carrier to the bumper, and the latch for the carrier. I'll post dimensions and pics.