Picasso Boy,
Check out the Ocean Rodeo Session harness. Can be used as a waist harness or with leg straps. The leg straps can also be integrated into a pair of neoprene shorts that you can wear under your board shorts. Not the same as a seat harness, which I recommend to my students, but the straps will keep the harness from riding up around your ribs. Also, I haven't experienced "leg strap" rash since I've been using this system. Check it out.
http://oceanrodeo.com/harness.html