After years of wishing my old workbench had a tail vise, I realized I could make one of my new Jorgensen Cabinet Master bar clamps do the job. All I had to do was reverse the sliding lower jaw (it's removable) and mount the bar and fixed jaw under the benchtop.
I drilled and chiseled out an elongated hole in the leg so the sliding jaw would extend only 5/8-in. above the benchtop. Then I lag-screwed a notched block and cap at the opposite end to act as a bearing surface for the fixed jaw. To finish my tail vise, I drilled a row of 1-1/8-in.-dia. holes in line with the sliding jaw and fashioned a bench dog to fit.
When I need to use the clamp elsewhere, I just remove the sliding jaw and pull the fixed jaw and bar out from the other end.