Add a strap under the belt clip base, insert your fingers and no more iPhone slippling off your hands!
Step 1: Parts & Procedure
1. Any iPhone case with a belt clip.
2. A strip of thin rubber, neoprene or even cloth. I used heat shrink tubing but I guess you could try other softer or more ergonomic materials.
3. Pliers to unscrew the belt clip's base.
4. Scissors or a sharp pointed object
1. Unscrew the base of the belt clip off of the iPhone's back.
2. Make holes on the ends of the rubber strip. The wholes should be big enough for the screw on the case's back to go through.
3. Put both ends of the rubber strip onto the screw on the iPhone's back (fold over the strip)
4. Now screw the belt clip base back onto the iPhone's back
Now slip your finger in the loop and use the iPhone with one hand without it slipping off your hands!
Step 2: Two Finger Loops Version
It is a bit more stable but I felt one finger worked better because of the ease of sliding one finger in and out instead of two.
You can now put the belt clip back on to the base and use it normally!