I made mine out of 1/4" plate and two diameters of square tubing so I could adjust height. Bolted to the bumper bracket holes. Have been on there for a long time and allowed me to transport the vehicle around in a trailer. They have started to twist a little over time, which could have been solved by additional gussets.

The adjustable height is good for allowing you to work on the vehicle while installed. Then you can lower for transport.

I would recommend pneumatic tires as it will allow you to roll over cracks or bumps much easier. Mine are about 10" in diameter For example the ramp up into the trailer, a solid wheel would have been really difficult to use. The guy that did my convertible top had a gravel drive, which also would have been impossible to move solid wheels on.