JR only makes one HD Nylon horn, it's roughly 3/4", part number JRPA215 and the covers will fit over them.

on the NACA inlets, as previously stated I've got a Titan SE in my Flash, never had a single problem with the inlets, I'd love to see pics of your friends who had problems. if you don't put the holes, you'll have cooling problems with the pipe and the rear fuselage area.

if you're really that worried about it, just line the inside with light ply before cutting the holes, then they would have a doubler, it's not necessary though.