David, the other alternative is, find a #0 factory choke from 18.4 bore, it'll be approx. #3 for your barrel.
When I "feel the need" to use odd # chokes in my 18.4 barrels, I just switch to 18.7 chokes, a #4 from 18.7 is #1 in 18.4, a #6 from 18.7 is #3 in 18.4 .