Looking for one and I have been for a while. Ordered a half dozen from Amazon for size, material and sewing inspection, all have been returned. Several have been ok on the last 2 but not the first.
Most were way too big or too small for an R-51 or a 3.5" 1911. The non-giant ones seemed sized to a .380.
Note-- I got the small size on this one.
I finally found one that is good with all 3 specs. Its just a half a hair snug on the 1911 but near perfect size wise on the R-51. Its also a good size to wear and does not look like a saddlebag or make me walk in a big circle. Quality and sewing along with internal support straps look good.
It has a second zip compartment which fits my phone and has room for some pocket stuff if wanted and will fit a second spare mag. Also a non-zip outside slot. Testing and carry over the next couple weeks will tell more.
Nemo
https://smile.amazon.com/gp/product/B08VMNVFM7/