I made one once for my old Duster with some 1/16th" thick, 1 1/4" wide (just make sure it's wider than your tach mount), and 10 3/4" long piece of metal.
I bent the metal in a thin, uneven U shape. So from the peak of the bend to the end of the metal on the shorter side was 3.5 inches. This is the side that is going to slide under your dash pad.
Make the gap between the two sides about 1/2 inch (inside to inside)
Mount your tach as close to the edge as possible(just over it) If you have a monster tach, you may want to add a small support bracket from the stock gauge cluster, or just use a little thicker and maybe wider metal.
I had a pretty big Mallory tach with the rev limiter and it worked fine without the extra support, but the monster tachs are a little bigger.