ahi, I am in the process of making one myself, two pieces of angle iron with holes in each about one and a quarter inches apart bearings out of a pair of roller blade skates, thjese are five sixteenth bore and about one inch o,d. useing threaded rod as spacers come axles, this allows you to adjust tracking for whatever gauge required, the roller blades came from a thrift shop, five dollars for sixteen bearings and eight rubber wheels for saving, and some aluminium for melting down.
Neville