Rusty, What you are looking for is an "air flow smoother". The size of the horns doesn't actually effect the tuning of the intakes as much as the mass flow. The horns just prevent the turbulence of the sharp end of the tube. I don't know of a source of more universal airhorns. You will be best served by a inlet with a full 1/2 radius torus, aka half the doughnut/bagel take your pick.
Bill Jepson
If I make a shoebox style like Tracy has, it will be too short to be "tuned", since the distance from the engine to the cowl cheek is only 12".
Has anyone seen a source for universal air horns? TWM sells a bunch of air horns, but they are made to bolt onto specific carbs. I would like to find some that slip over various sizes of tubing if possible.