I've pretty much finished my carrier, with the exception of painting it. I've added a few things as you will see in the pictures. First off, I turned the bolts down so they look like pins instead of bolts, and cut them to the appropriate length. I added 2x10s, drilling the supports to accept 1/4" carriage bolts. The finished width is 40".
I put 1/8"thick by 1 1/2" steel around the edges of the wood to protect it, securing it w/ 1 1/4" #12 wood screws. This should make the wood last a long time. I then drilled a number of 1/2" holes to provide attachment points for straps or bungi cords to help secure things I haul.
I thought it might be nice to be able to pull a light trailer, (I just happen to have one) but I didn't want a hitch sticking out the back to wack my shin on. I also didn't want it to be so low that it would be below the angle supports. That would make it difficult to put a pin in, so the solution was a bit unusual, but will work. I drilled a hole through the wood on the carrier and will fabricate a nearly flush headed pin that will go through the wood and pin whatever trailer I choose to pull. I considered welding a cross piece to attach the hitch to, but decided to try an experiment. I used a 3/8" thick bar 1 1/2" wide, drilling and attaching it to all three boards with 7/16" carriage bolts. I think it will hold up, but if it doesn't, I can easily add steel cross members.
My daughter, Spencer, took it on its maiden voyage, going over to my folks house to retrieve a bale of straw... not too much of a test, but she didn't tear it up, so there is that. I then hauled my son Carson and a 193 plow and lever back over to the barn on it and my son drove back hauling ME. (so now I know it will haul 210 lbs and not break!)
When I DO paint it, I'll add that pic. I think I'm going to paint the steel blue and leave the treated wood alone. We'll see how that looks in a few days.