More then a while back, I went rock climbing with a climbing harness consisting of two overhand knot loops and several turns around my waist secured with a reef knot. While not particular comfortable, it was simple enough for a bunch of untrained teenagers to reliably tie. You can find it about half way down the page under "Improvised seat harness"
http://www.wilderness-survival.net/mountain-climbing/roped-climbing/ I have also climbed with that rig, and my memories are not too fond. Our practice was to finish with a water knot, however. It probably doesn't make too much difference if you have enough turns around your waist. Today's commercial harnesses are money well spent.
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