I've been trying to design midrange and HF horn flares to be built out of fibreglass. The midrange driver, using an 8PE21 as per Xtro is causing me some headaches, mainly regarding the throat/phase plug section. The idea is to use a circular or elipsoid conical horn with the Peavey designed quadratic throat (
http://aa.peavey.com/downloads/pdf/qwp1.pdf). This would be simple with a non-phase plugged horn and should work well on the 1" HF but it's a bit more tricky to maintain the flare rate with a phase plug in place.
Another thing I'm worrying about is HF beaming. With no phase plug, if the horn has an length of 30cm and a 60deg dispersion, the curved section will extend halfway down the horn before becoming conical. Could this cause beaming at the upper end of the horns range? Adding a phase plug may then require a section with parallel outer walls, I'm thinking that's not such a good idea, even with a phase plug in the centre. Having said that, the midrange on the air motion seems to do something similar. Without having intended to do so, it does seem like I'm trying to do something very similar to the Air Motion mid and top. At least we know that sounds good, so can't be a bad way to go!
I will try to do some drawings to post to make it easier to describe where I've got and where I've hit problems.
Construction wise, when I can get some foam to make moulds out of, I might start with a copy of the Xtro horn in fibreglass to use as a benchmark for any new designs.