I gave up on that and now carry it in a serpa holster.
Now I understand why your gun fell out.
- By design, that holster style gives up about half the retention of a normal holster since there is no friction on half of the slide.
- There are a whole lot more poorly constructed Yaqui slide holsters than well made version.
- Because the muzzle is exposed, an impact on the muzzle will force the gun out of the holster, such as even just sitting down in an arm chair and banging the muzzle on the chair's arm.
It is a good design for a training class on the range since there is so little friction on the gun, but not a good choice for concealed carry. If that is your sole experience with an open top holster I can see your skepticism with the security of open top holsters.
You can do a google or forum search for folks looking to break in their leather holsters or for the "plastic bag trick". They can't get their guns out of their holsters and are trying to get the holster to the point where they can draw the gun without giving themselves a "wedgie". Most well made open top holsters provide quite a bit of retention.
The basic design of a pancake holster also provides retention. The classic design uses two pieces of leather of the same size. When put on the belt, the outer layer has to bend more to cover the longer distance and somewhat collapses, providing retention to the gun. This design often doesn't work for the very slim user as the bend is so severe, the gun almost cannot be drawn out of the holster.
If you still need a retention device, make sure you get some feedback from folks with hammerless guns to make sure a thumb break works for those guns. Otherwise, I suspect a thumb break holster, that doesn't work, provides less retention than an open top holster. The thumb break holster is counting on the thumb break for retention and not the holster body, while the open top is using the design of the holster body to provide retention.
Safariland probably offers a holster that provides retention at a different point than at the hammer. However, I'm not that familiar with the LE retention devices to know how they work with small revolvers.