You don't really have a "classic" Herbie overflow. I agree it's more like Bean's which is really just a modified Durso with an emergency drain.
A Herbie has drains on the bottom of the overflow box so the water can only flow one way: down. There is no air introduced into the system.
sphelps's solution should work, but you'll basically have a Durso which is a controlled way of introducing air into a drainage system.
Here's my Herbie:
