Can't remember which set of vintage reprints I saw it in, I think maybe the trailer for two or the honeymoon design, but didn't one of those designs build a flat platform and drill holes thru the lengthwise stringers for a model-T axle instead of doing a steel frame?
Saw a pic somewhere of wheels attached to a treated 4x4 too, but don't remember where it was now ...
But I definitely remember wood frame and, I think bent iron pipe for the a-frame/drawbar ...