This was the 60's (yea, I've been doing this a while), but several manufacturers used the 30 watt EV or similar reentrant outdoor paging horn in a textured black painted particle board cabinet with a simple "horn loaded" 15". The 15 set totally in the straight wall "horn" mouth that was maybe 6" in length, so there was little if any horn loading. The cabs were heavy, the edges crumbled with impact, and the entire cabinet self-destructed if they got wet. Did I mention the outdoor paging horn? You know the sound!