My wife carries a Spyderco Delica, and I carry a Native for our using knives. They work fine, and are easy to replace if lost.

I also recommend the Benchmade Griptillian with the opening hole and Axis lock as a non-threatening looking knife.

None of these have the open frame feature you mentioned. I notice that the CRKT S-2 is still available in two sizes. This is a great frame-lock knife, with an open frame.

Bear