I have two S/C holsters. One I keep OWB for the range, and the other I modified to work for me in an IWB fashion for my Glock 26. Total, not counting a little time, I have about $22 into this holster. There is a company local to me that makes custom leather and nylon police gear. I walked in actually looking for him to add a leather belt strap to the holster. That way I could keep it secured to my belt. What I walked out with was: the use of a metal mobile radio holder clip installed. How it works is one part of the metal clip is slid inside the holster, and locates into a oval hole we punched in the plastic of the holster. It is VERY secure, and definitely not going anywhere. What I then did was cover that metal (and basically that whole side of the holster) with commercial grade, super sticky soft loop velcro. This prevents the gun from sliding in and out on the metal clip. I wish I would have taken some pictures of what the clip looked like out of the holster. Because its not coming out now without ripping out the velcro.
Heres some pictures. I also modified the holster to compensate for my extended mag release, and extended slide release. I show a unmodified holster in one pic so you can see where I cut it up a bit. Smoothed it all out with a Dremel tool and sand paper.
I couldnt get a good picture (crappy cell phone camera, and poor lighting) of how the inside looks, but it just has velco on one side. It makes for a slightly snugger fit, which I like. I sprayed down the holster inside and out after (not the velcro) with silicon spray. The holster hold firm, but releases smoothly. With the clip at the right cant, and how it attaches, it allows for very deep concealment. Let me know what you think! I'll try and get some more pics wearing the holster soon. I need someone to take the pictures. But its comfy at about 4 o'clock, or slides in easy for appendix carry also.