I'll second the warped sense of humor.
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....my belief that you can rappel on almost any sufficiently irregular piece of metal that has round edges and a hole to clip in.
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I'll go further that many devices don't even need a hole to clip into, like the Titan, for example. Run a bight of rope through the slot, run you connection (carabiner) through the bight and you have a friction controller. As long as the slot does not allow the biner to pull through, or for the rope to get pinched in any way between the device and the biner, you're creating friction.
Round edges are key. Sharply angled edges, bad.
The device (technically) does not even need to be physically attached to the harness or connection. Take the chain link, or friction saver ring for example. Run the bight through, clip the biner through the bight, that's it. Gravity and the second bend in the rope (coming vertically up and out of the slot and back down) keeps the piece right there. Now, if you climb above your tie-in point you lose that second bend and the piece falls below you and out of reach, in accordance with gravity.
Again, don't use a chain link. An oval quick-link, stainless steel, is longer in profile, thinner in material diameter and works better. And is rated.
I have a design that just came to me recently, based on a quick-link and a rope thimble. Instead of the bight going through the quick-link and around the biner, the bight would go through the link, then pass over the countoured, 'teardrop' shaped thimble and then back through the link. Your connection would then go around the terminal bottom of the thimble. I believe the thimble can be successfully welded to the 'nut' on the screw-link, allowing you to open and close it to apply the rope. Here's a rough draft (very rough)....,
