I am bringing this thread back to see if any newer info has arisen to finding a shorter extension tube.
The reason I need one is so that I can stack MKIII teleconverters together, since Canon changed up the design enough that the MKIII 2x doesn't like being stacked with itself or the 1.4x III. A tube in-between should fix this issue, and I want as thin a tube as possible to reduce the "lost infinity focus" effect.
I thinking about trying to make one from a Kenko tube, by shortening the body and putting the mounts back on, without disturbing the ribbon cable, or whatever is there for the electrical connections.
I will try out my 12mm with the 2x on a couple of lenses that don't mount to the TC today, just to see what the impacts are, but I would love an 8mm if there is way to buy one or make one.