The fabric was affixed to a wood frame to form the mouth, then a wood disk (from the throat cutout) was centered on the fabric and pulled from behind to stretch it into a horn. The frame shape and some strategic stitching gave the desired flare. The Lycra was then coated on both sides with layers of resin and fiberglass. Once dry, the throat was adhesed to the frame and the excess material was cut away.

The added bonus is that your horn can be any color you wish. Very clever!