When I run, or on days I just wear athletic shorts, I usually tuck one of Spyderco's FRN knives in my waistband. They are for the most part very thin. I've honestly ran with a range of FRN Spydercos in a variety of steels, and while there usually is some moisture on the blade, I've never had an issue with rust. They also tend to be very lightweight. So I'd say pick any design FRN Spyderco you like, and go with it.