I first heard of Martin Seddon's fiberglass horns
in the context of his AH-160
drivers. Martin had used the (acoustically sweet) horn curvature developed and published by LeCleac'h to make molds for fiberblass layup. Martin very generously described his whole design/build process to me in email, but he recommended that the mold-making process had been a royal pain in the ass, and he recommended that I buy horns from him rather than make my own molds. This seemed like good advice, but it stalled me out for a few years.
Once I had the money together from the sold-off salvage, I immediately ordered a pair of his AH-425 horns, which were designed for the remaining pair of salvaged Altec 288C drivers
The horns simply bolt onto the drivers, so there's nothing to "build" here.
Those drivers on those horns cover (conservatively) around 800Hz to around 8,000Hz, so I still needed to cover 30-800Hz and 8,000-20,000Hz:
(The photo for this step is from Martin's website.)