The keys to appendix carry are a: physical fitness, short barrel, holster that has minimal material in the area in front of the trigger guard and beneath the bottom of the front part of the frame, and vertical and horizontal positioning. An oversized shirt helps too.
I occasionally carry a Glock 21 appendix. It's not that difficult. The primary problem with it is the long barrel digs into my thigh or abdomen, so I can only stand it for a few hours. I have no problem with a SIG P220, but the SIG P220 Carry is even better. I have found the Glock 19 sized guns to be a great compromise between size and comfort.
Glock 21 AIWB holster on the right
A Kahr PM9 is very, very easy to carry in the appendix position, as is a short barreled revolver. I frequently place my backup gun in the appendix position for two gun carry.
Jeans make it a bit tight, so you'll need some extra room. I prefer pants that have a bit of elastic in the waistband. It makes wearing the gun a bit easier. A very thick holster (such as a leather holster with two layers) hurts concealability. Additionally, if the loops are on the face of the holster, the overall thickness will increase by up to 1/2".
Width doesn't matter in a properly designed holster. G21 versus SIG P220
My favorite appendix carry rig: Glock 26
Printing can be a problem. I have found the loop on the front to be a big offender. However, the end of the grip (usually the magazine) often pokes out a bit. If you position the gun correctly, it will look like a phone or something. The trick is to make the "bump" appear inconsistently, rather than as a "ledge" that oultlines the back of the grip. Short grips, such as on the Kahr PM9, Glock 26, and J-Frame revolvers are not a problem. Adjust the gun right or left until it disappears.
S&W 625 N-Frame revolver with 4" barrel. Slight bump on the right side from the stocks.
Two gun setup