They have a split rim that is pulled together by something similar to the catch at the bottom of a Miller rim. They occasionally come up on eBay or similar sites and you may find them at an autojumble. If suitable rollers were available they could be made from brass sheets and plated. Potential sales would likely be too small for a company to make a batch unless you know "someone".Are the converter rims available or do you have to beg them off existing owners as it seems like a sensible mod to me.