Looks great and easy enough to build out a few 2x4, and I'd got those air filled "tars" you can get at Northern, Harbor Freight etc... about $10 a pair...just fashion an axle.
That' something like the one I built for my S-I-L at the farm. On mine however, the PHD hung from a chain lift on the cross beam and had bigger tires. It also had a 3-step ladder for my S-I-L to get up to it ... she's a smurf.