Boy now if that aint the question of the day! I have an Uncle Mikes nylon holster like the one pictured that would probably work. I have a nice one for my OWB carry of my P220R45 full size SAO. I know that for now, my IWB carry options typically include the compact or sub compact Glock pistols, and seem to favor my body type. I have alot of experimenting to do concerning the larger brands of "carry or compact". The SIG grip, the slide, and the 'cocked and locked' hammer spur dictate to me the need for something between myself and the end of the slide/hammer of the pistol. I have searched the internet, and the closest thing I might ever attempt to do this with would be one of the vendor/supporters of this site--
CrossBreed Holsters, Innovative holsters for concealed carry-1911-Glock-HK-Sig-Taurus-Kimber-Springfield-Colt-XD
if they indeed make the holster for that model. The only other option I could think of would be the kydex type:
Scorpion IWB Holster
I love my new 'Scorpion' for my G27 I purchased from one of the members here. Good luck, and kudos to you concealing IWB with the P220 w/rail!