Inspector
Nyuck, Nyuck, Nyuck!
Guys that make their own speakers use them. Try speaker ports. 150mm are hard to come by but are out there. The Aussie forum shows how to make them by heating PVC on a form in the lathe. Later when I have time I can link it to you if you want. Or do like that one and turn it from MDF or wood on your lathe.
Pete
Pete