I had the same problem when I converted to 14 inch tires
and I looked at all of the solutions offered by factory made tire mounts. I purchased one like the one in the photo that Joe posted but the tire hung so low that hit the pavement when going over any small dip. The solution was very simple. I just cut a new disk to replace the one that Casita
made from scrap fiberglass and drilled the hole off center to compensate for the larger diameter tire and wheel. I made the new disk out of an old plastic cutting board.