I have used a runner, knotted in the middle and re-clipped to my harness's belay loop with an extra biner, to extend my rappel device. I never get the knot in the right spot. I also use the biner clipped to my belay loop to anchor in once I hit the belay station. It's never the right length! Grrr...
This fixes all that AND it has no dyneema weakening knots tied in it.
It's 16mm of dyneema and nylon badness.
BUT, please girth hitch this to your harness's waist and leg loops and NOT your belay loop.
AND clip in nice and tight (you can even adjust your clip in point with an extra biner without ever completely unclipping from you anchor) and avoid those nut busting factor 2 falls.
I took away 1 star because all those stiff stitched sections get to be a pain the butt sometimes.