Phil-l
I think you imagine it pretty accurately.
I first secured the inner baffle to the upper piece using a few scattered daubs of RTV. (My retaining clips had already broken off before the first repair ... they were waiting for me atop the spacer). The two parts were pressed together and the RTV allowed to cure a bit.
On installing the upper (two-part-assembly) to the plastic spacer: I first did a sealing experiment on a single horn ... allowed some cure time ... and then pulled the upper away from the spacer to evaluate the seal. (You're right. You can't see what the results are without disassembly). The final method was to run a RTV bead in each female "nest" and also a mating bead around each male horn tip. You have to work pretty fast to get all this done before curing begins ... especially on a warm day. The bead diameter was approx 1/8" - 3/16". Using my finger, I trimmed the beads to what I felt was a shape that would surely seal the joints.