I have made a few,one using the FWD car hubs and space saver spare tires with pipe for the axles like Small Fry did..one I used a rear axle & wheels off a Dodge Caravan my friend was going to scrap..kind of wide,but you'll never hurt it!..
Another heavy duty one I made by cutting a 275 gallon oil tank in half and used the rear half of a rear engine riding mower for the base and wheels,made that one a dump style..pretty heavy though even empty,its not good for a light lawn tractor,it'll lift the rear end off the ground when heavily loaded if you use it on a small tractor with no wheel weights ..
The handiest one I made isn't a cart at all,but is instead an old MTD lawn tractor I cut in two at the tunnel just before the seat and added frame rails made of street sign square tube with the holes in it,and I made a pickup bed style dump body that can slide into those rails ,it can be dumped while sitting on it simply by lifting it up,and the bed can be removed when not needed when you need to tow or manuver in tight spots..was a bit "butt heavy" till I added weight up front,steering was a challenge without it,it tended to do nice wheelies!..
I've seen a few regular wheelbarrows coverted to a one wheel trailer too,or you can add another wheel to one to make a wider axle..one guy down the street has a large gray warehouse plastic bin thing that used to have casters he put wheelbarrow wheels on using water pipe and 5/8" bolts for an axle,just u-bolted it right to the bin..he says it sucks to unload though,its pretty deep and has no "tailgate"...carries a lot though!..
