There's nothing wrong with using fiberglass as long as you use a vent screen to keep any dust from exiting the cab. Regardless, you should cover the vent anyway using however much density is required to critically damp it when tightly stretched across it, i.e. remove any hollow box sound and echoes from the back side of the driver that the wall damping may not have attenuated enough.

FWIW, I had the same issues WRT small Vs large format horns and found that two 511/802 combos stacked on top of each other yielded the 'full' mids of the larger with the 'sweet' HF of the smaller plus a bit better overall polar response.

GM