The towers don't use a vent - It's just a cutout in the baffle that forms the port. We're actually looking for a large diameter vent that is very short, so the baffle width itself sets the length.For 5/8" wood stock, cut a 4.5" diameter port hole.
For 3/4" stock, cut a 4.625" hole.
For 1", cut the hole 4.75" dia.