About 2000 years ago, I fabricated one out of leather with a much wider belt loop for that very reason.
In the intervening 1999 years, I never really found anything suitable except something I am currently using with one of my set-ups:
https://condoroutdoor.com/191045-water-bottle-pouch.html
It’s not as sexy, but it fits the Folstaf perfectly, you can fold the flap inside of the pouch if desired and you can adjust the opening & retention with the shock cord drawstring.
I did find the shock cord drawstring a LITTLE too tight for my liking so I replaced it with a longer length piece of shock cord so I could open it up wider. That makes extracting or putting the Folstaf back much easier.
It just so happens the set-up I use it with is designed to utilize the PALS/MOLLE attachment system, but the pouch comes with long MOLLE straps & PALS webbing so it would be easy to have it on a regular belt without much slop.
There are also MOLLE belt loop adapters out there if you want even more flexibility. I use
this one with one of my pouches on a regular belt.
Good luck in your search!