That photo of installing the phenolic horns doesn't show much of a pad for a bolt on horn. Ithink most of the nylon horns would end up with one or more holes having nothing but covering under them. Perhaps one of the Dubro heavy duty horns might work,
http://www3.towerhobbies.com/cgi-bin...&I=LXEMF9&P=ML
Just harden the drilled hole with CA or you may squash the wood when you tighten things down. If you do use this style, I would be very temlpted to cut the phenolic horn off at the covering line and glue the stub into the slot, Then add the new horn. This would tie the pad all together.
Don