Take a look at Blade-Tech's Klipt appendix holster. Very thin and comfortable, and can also be worn cross-draw or behind the hip.
I have a Blade Tech Nano IWB and a Dale Fricke Archangel for my large guns. Both are top quality 100% Kydex holsters. They both conceal well and are stable. They both are uncomfortable for all day wear because they concentrate too much pressure at the bottom of the holster. I have added moleskin down there, but that is only a marginal improvement.
The challenge is the large, heavy gun. The Shield is fairly easy to conceal because of its size and weight. I'm using a simple, small Galco single plastic clip positioned in the middle of the IWB leather holster for it which works OK, but it is cheap and I don't expect it perform well over time. The gun sits too low so I have a paper towel folded in the closed bottom to raise it up. Hokey.
Any design that would work for a full size gun should also work, somewhat scaled down, for the smaller gun.
I'm a fan of leather. Kydex works great for OWB, but it's stiff IWB characteristics are less desirable than a larger leather IWB contact surface, IMO. Kydex's main advantages are relative thinness and strength, open for reholstering without reinforcement. After that, it's characteristics are negative IMO.
I refitted a 100% leather IWB custom holster of mine to 0 cant and found it getting uncomfortable by the end of a day, again because of a concentrated contact point at the bottom. That is where the pressure needs to be spread, especially if applying some kind of wedge (good suggestion) to push the grip in tighter to the stomach.
I have been dealing with the discomfort and concealing fine. It's just that I believe a larger leather backer will stabilize and transfer the weight of the gun over a larger area giving greater comfort. I typically wear my EDC 14-16 hours/day.
I have been wearing concealed for over 20 years, most of it very comfortably with mid size or compact guns in Thunderwear. It's drawbacks are a somewhat slower, more restricted draw, and they wear out pretty fast with the heavier guns. Access is particularly limited when seat belted in a vehicle.
A single clip at one end of an AIWB holster doesn't make sense to me for stability or spreading the weight, even if it is a Shield. The vertical center of gravity is about 4" from the clip support point. As well, if that clip is at 1 o'clock, the gun will be at 2 o'clock, farther west than is comfortable and concealable for me.
I own other leather IWB holsters by Milt Sparks and Mitch Rosen which have always worked well, but they are for behind the hip and with cant. In a trade I got a nice M&P FS RMR Kydex holster--with a 15 degree cant. Can't get it to work for AIWB.
The point is I've got a fair amount of experience carrying concealed with some good gear. I believe there is a better design available, but for whatever reason no one has put it together yet commercially.
