Just revisited here, and another year later, so far none, but what you link here shows precisely what I was after, and is a really nifty DIY job from scratch.
A problem for me is that the rear rack of my avatar bike has a poorly designed connection to the seat tube.
It has two steel tubes or rods to adjust the distance, each a clamp to fix the position but the clamps are metal on metal, have proved themselves to slip (and not due to losening bolts because those are nylon locknuts), and there are also no stopping ends, so when they slip over the ends, the entire rack tumbles backwards crashing on the road.
Last year at the bike shop, by coincidence, another customer walked in, having suffered precisely that. The rack had slipped off, crashed, while riding, holes were grinded through the back of steel tubes and the rear Son lights coax was ripped of, all together a costly repair and the customer decided to let it like that.
As a precaution, I connected the frames front with a belt to the seat tube, so that if it looses again, at least it wont tumble over throwing everything on the street.
I would need to first try another, better designed rear rack, in meantime I found a few, also very cheap, but due to the broken frame and the replacement hassle, didn't put time in it so far.
Its very nice to see an idea already having been worked out before; mocked up from scratch, and proven to work.
A versatile lightweight bare bone thing.
That ladder auxiliary frame was thinking on top of thinking.
It brought me some motivation to rework on the idea.
