I don't think they make them anymore, but if you could find a used "Safariland #42 LGE. S.A. 4 5/8" stamped on the rear, you couldn't do better for a high ride hip holster for your Blackhawk. it has a retaining strap, but no flap. Coincidentally, I had my Blackhawk .45 L.C., in that Safariland, on my desk as I read your post. I've got excellent service from mine, for years.
If you must have a flap, check with Kramer Leather, as they make a dandy. But it's large, so don't expect any concealment, if that is what you're hoping for.
Good luck. J.B.