My new "Road Bike" can best be described as a hybrid. Cyclocross frame, fork, wheels & tires, with a mountain bike drivetrain. I'm a fan of bullhorn handle-bars, most comfortable/familiar with SRAM mountain trigger shifters, wanted to give reverse levers a try, and didn't want to spend the money on road combo brake/shift levers.
I knew the new(er) SRAM shifters have detachable clamp, which gave me the idea to find a way to use them. I picked up a pair used, on ebay for $35, which is about 50% of the street price. The total price for brake/shift levers is about $65, compared to $200 for an entry level road combo lever.
I tried finding a larger clamp, for this purpose, but had no luck. I had some 1/2" thick aluminum in the garage, and ultimately the clamp I need is a refinement of a piece of aluminum with a 23.8-26.0 mm hole in it.
That's how the machining (on a drill press) started - I drilled a couple of 1" holes in my 1/2" aluminum plate. I added the through-hole and counter-bore for the clamp/fastener bolt. Then I trimmed away some excess material.
It's not finished. I still need to make a groove where it mounts to the shifter mechanism (keeping the shifter from twisting), trim away more material, and generally clean it up. But a quick mock-up illustrates that it's on it's way to doing the intended job.