I have a brand new, never used IWB Bordertown Holsters holster for the Gen 5 Glock 19. It looked pretty close, so I shoved it in and it's workable. The last little fraction of an inch has a little resistance instead of locking in like their holsters usually do with the correct firearm. It's close enough, trigger guard is completely encapsulated and there's no protrusions that can play with the trigger.
I looked around and saw a sad and lonely Glock 43 magazine case, also from bordertown. I looked at the construction and realized it I could heat it up and reform it. 2 minutes with the hair dryer with the Hellcat magazine inside, and it's ready to go.