Depends on the maker sometimes. Some list the same part no. holster for Colt SA & Ruger BH. Others? I have a couple of the mid frame FT's & they fit nicely in newer or less worn holsters for regular BH's. Thinking they would be loose, but not in my case.
If I were to order from a maker that had different holsters for Ruger & Colt I would order for a Colt New Frontier if they listed it. If not, only a Colt SAA or Ruger BH, I'd order the BH holster.
I don't intend to make it sound so complicated, but I went through the same thing, wanting a holster that fit really well, & not seeing any listings especially for the flat top.
When you get right down to it........a holster for a regular BH should be fine. YMMV