Howdy, Pard,
The best thing you can do to keep the gun from falling out of the holster while you are moving is to have a flap or a hammer thong that you leave on the hammer spur until you are ready to start the draw, or at least until you have stopped moving to the firing position. This will slow you down some, but with the short barrel it is the best way.
So far as the holster design is concerned, having the holster cut high enough so that it encloses both the trigger guard and the cylinder and frame, and perhaps high enough in front to shroud the hammer, will help. I prefer to make all my holsters with a closed toe, either with the sides simply pinched in and stitched or with a teardrop-shaped plug, to prevent dirt from plugging the muzzle if I sit down in dirt, mud or snow. Closing the toe will help the holster keep its shape, especially when the holster is properly wet-fit to the gun. But eventually, the leather will stretch a bit, so that I would NOT depend on it to hold the short-barrelled pistol in when moving around.
So far as there being a "foolproof" pattern is concerned, there really is no such thing. What you do want is a holster that hangs straight up-and-down on your strong-hand side, with the gun butt to the rear (conventional draw). I presume you have an "Old Model Vaquero" with the 3-3/4" barrel (measured from the front of the cylinder), WITH the ejector. That limits the holster design a bit, as you have to have enough length on the body to accomodate the belt loop. While you might get away with a "Mexican loop" style, the body of the holster is a bit short to have enough room to fit a skirt band (where there are two slits in the skirt that the body slips through), and even the later style with a separate skirt band lacks enough room. Best bet is a "slim jim" with the belt loop sewn on to the back of the holster. Of course, it depends on how wide a belt you are fitting it to.
If you like, you can e-mail me at trailrdr@ecentral.com and I'll try to give you some more information.