I have only made holsters for myself, but I had to "ease" the pattern for an SAA/clone by enlarging the dimensions about 1/8 inch on both sides. There is about 1/4 inch more "wrap-around' required for the Old Vaquero. I have one New Model Vaquero which fits a SAA holster, only a bit snugger than a Colt or clone.
As mentioned above, an SAA/clone will drop into (and maybe out of!) an OMV holster. but the OMV is a VERY tight fit for a SAA holster.
Wetting an ill-fitting holster can work both ways. Make it bigger by inserting a rust-protected gun or dummy. Make it a bit smaller by letting it dry in a warm place. Either way, the leather will have to be re-conditioned afterwards. Lined or double thickness holsters are very difficult to alter.
Sometimes, depending on the pattern of holster, taking another line of stitches inside the original line might make a loose holster useful, but may not look too good.