I hit the jackpot and got a Jarno 12/5C adapter off ebay in really good shape, so I needed a 5C collet closer. I ordered 24 inches of DOM tubing and a 6 inch slice of aluminum from Speedy Metals. It turns out that 24 inches isn't quite long enough, 26 inches would be better. Just enough space for a machined end sleeve and thrust bearing. If if turns out that I can't grip well enough I'll make another, metal is cheap, but I'll see how this works for a while.