I built a hoist "mono trolley in my garage. I used 2 x 6 on edge with a strip of steel on the top. I used a solid wheel from a HD store caster. It works great for me, perhaps a variation might be useful for your project.

Also note in our state we have a company that sells steel commercially. They have a large out building with "scraps" (some are really big scraps). Perhaps there is one in your area.