I made one for a Model 29 Smith and Wesson back in about 1977. Retention was an issue I finally overcame with a thumb snap. It worked, and concealed great.
Right on the money, Mike (56? You're still a bub). Retention was an issue with the upside-down holsters, beginning with the original Berns-Martin:
It's not entirely clear when this holster came on the scene, because the mag articles I've seen from the 30s speak only of the famous Speed holster; and it is in reality a conversion of the common shoulder holsters of the era, into a belt holster. My original catalogue is from 1966. These use a spring that retains above the cylinder. That three-stitch plug is there near the muzzle for a reason; and its omission is why the elastics don't retain. But the spring is not very strong.
The Bianchi 9 originated in the 60s, the oldest record I've seen being from 1963/4 when Perkins and Bianchi were a team; when Perkins split off, he made his own version as in MSGT's post. The Bianchi was copied heaps.
The Bianchi 9R, one of my first assignments as a youngster at Bianchi, was originated because the elastics don't retain under certain circumstances; which I will explain in a moment. Turns out it didn't truly solve the retention problem -- until we added a rubber grommet and screw assembly that emulated the leather bit in the Berns-Martin mentioned above. Notice that in this pic there is no screw assembly to act as a pivot, and the revolver has arced its way loose from all retention.
The 9R2 was created to provide a hammer-spur guard, as in the Bucheimer "Marshall" belt holster; and make it a dual-side belt holster; and somehow we were back full circle to the Berns-Martin "Triple Draw"! Except retention, and durability, were excellent. Note the post and screw assembly near the muzzle.
All shoulder holsters need to pass the test we created as a tool to solve the retention problem in the 9 and early 9R: it's called the "snap test". It was created to simulate an officer who had jumped a fence and found himself without his revolver; when he landed the jump, the revolver came out.
With your revolver in the holster (only leave it loaded if you want an a/d, your choice), grasp the harness at the centre of the shoulder strap, and "snap" hard. Hard. Many shoulder holsters will give up their revolver, so do it over your bed

If the revolver stays in, then you have a good design in your hands. If the design is poor, you have a good revolver on the floor.
"Those who cannot remember the past are condemned to repeat it.", which is why there were and are so many copies of the elastic version. The spring versions are beyond the hobby craftsman, though I made my first at home when I was 19; trying to copy the Hoyt (
"I didn't try. I succeeded". Sheldon Cooper).