I've had some quarantine time to work on this project. Finished cleaning the H1505 up, figured out how to build and mount it onto the top plate so it doesn't actually modify the 30W5 while still fitting securely, and putting together semi-old hardware for adjustment capability. The bolts are mounted through the plywood veneer for strength, this horn-plus-driver-plus-plywood weighs north of 150 lbs.
Attachment parts are not original Altec-Lansing, but neither is the top plate. My goal here is to be able to assemble a working system so I can hear its basic sonic signature, leaving the original parts intact for a future collector.
I built the gaskets out of new cork rubber, found some old 5/16"-24 bolts for the throat, and some old '60s vintage fiber-rubber gasket material that matched the horn flange thickness exactly.