About the woofer cutout hole, I don't think its shape or proximity to the apex are terribly important because the frequencies you'll be using the horn for are so low. Cut it round or square, whichever is easiest for you to do. I would position the woofer(s) near the apex and cut a hole for each woofer, located centrally, just over the dust cap. That will make the hole less than a foot away from the apex, which will be just fine. The wavelengths you're dealing with are much longer than that.