Wheel Adapter

Maybe get a 14" BBP wheel instead of an adapter.

Just keep in mind if you have a flat on the rear running two different size tires for any distance can damage the rear.

Have you looked at maybe some kind of small tire to go on the 15? I'm not sure they make such a critter, but it might be worth looking for. You would have to measure your well and look at the dimensions of the tire.

I did some looking for the adapters and you can find the small to big all day. The other way around was elusive, lol.

Do you know what the tire height is that you need to fit the well?