You want to carry truely in the small of your back? Or just behind your hip? A 4 o'clock position is very comfortable and provides decent concealment.
A true SOB holster prints badly when you move, makes sitting in a chair or car seat extremely uncomfortable, often encourages you to sweep your body with the muzzle when you draw, and can cause very serious spinal injuries if you should fall and land on the gun. That's why many manufacturers have very few offerings in their catalogs. Some won't make them at all.
I use a "pancake" or "Speed Scabbard" (same thing really) OWB holster to carry my 1911 most of the time, as it is very comfortable and the smooth outer leather surface blends the gun's profile smoothly beneath a cover shirt.
If I'm going without a cover garment -- or just want maximum concealment -- I use a custom tuckable holster very similar to a Versa-Max II. I've also had fine success with a Summer Special IWB.
IWBs are extremely secure. The gun is in tight and usually carried deeper than an OWB -- and your belt tension can be adjusted to make them as tough to remove as you want. I ride roller coasters, have been in roll-over car wrecks, wrestle with my kids, climb trees, etc., etc. with any of these and have never had a gun move a hair out of the holster.
A thumb-break or other retention holster is designed more for openly carrying where you might want to slow down a gun-grab or might have you gun jostled loose from an OWB holster. I can only imagine that the few thumb-break IWBs that exist are built for police officers whose departments mandate they must carry with a retention holster.