Find a good saddle maker. Most of them can make what you want and do a very good job of it
I know 2 top notch makers in Riverton Wyoming that can make anything you want, and their craftsmanship is outstanding.
Shipping the gun to them is the problem. They form the holster to the gun, and they don't have FFLs. So if you can't hand them the gun, it's not possible for them to do it for you.
But, Iowa is full of horses, and so there have to be some good saddle makers around.
Look in the phone directories.

Steve