I have a leash that is super long and it has clips so that it can be worn as a belt. When using it as a belt the leash is about the perfect length for my old dog. However I never use the belt part of the leash because I use another belt to carry the poop bags and other accessories necessary for walking my dogs. Therefore, I needed a way to secure this leash around my belt. 

Using this plastic bracket I thread the leash through the bracket and it normally just lives there (Picture 3). When I use the long leash I loop the leash around my belt and then thread the other side through the bracket, securing the leash in a loop around my belt (Picture 4 & 5). The excess leash then just hangs down my thigh.

I have been using this for several weeks and it works really well. My old dog pulls like a mother when we start the walk, is dragging behind at the end of the walk, and the leash always stays in place. Its easy to put on the belt and take off. Version 2 is squared off but my partner didn't like it because the sharp edges would catch her when she used the leash as a belt. So,  I made Version 3 which is rounded off on all edges and this one has been approved for regular use by the boss. The leash I have is flatish, 25mm wide, and this bracket is specific for that width of leash. 

I printed the square version (2) without supports and 50% infill. Version 3 I printed with supports and 50% infill. The supports may not be needed but they were easy to remove and they help to take off all of the offending edges. I printed this in PETG.