Courtesy of Didacus R. , Haywood, CA
I found this link to a bike trailer/dolly. My only modification would be larger wheels placed so they are equal to the bottom of the trailer when it is upright as a dolly and far down enough so that they don’t interfere with the trailer bed when laying flat as a trailer. Of course that part is not critical if you don’t intend to load your trailer wider than the width of the trailer.
https://www.gizmag.com/convert-bicycle-trailer-cargo-dolly/25644/
