I know I've mentioned this before elsewhere, Make sure you coat the inside of the horn with something that will go hard. The first wooden horn I made years ago sounded ok when finished, but it wasn't bright enough until I coated it with Fibreglass Resin. Something like Ronseal would probably do the job. It's well worth doing. If you don't, the ply will soak up HF.
Peace and goodwill to all speaker builders
