So, in TMP, we see that N Deck, the level above Main Engineering, has a hexagonal opening at the top. In the shot, we're actually seeing that hex opening up into the soundstage, as there was no ceiling constructed for the set, AFAIK (one may have been built for this area in TNG, but I'm unsure and I don't feel like hunting for a shot).
As a matter of fact, there was no intermix segment constructed above the one that is most prominent on this deck, as you can see that there is only a single "donut" above this warp core segment and isn't doubled, like all the segments below it.
However, we know through Probert's schematics and MSGTTE that the intermix shaft continues upwards and meets the impulse deflection crystal. It is also commonly assumed that inspection rooms surround the intermix shaft at each level above N Deck, much like they do below. Given the architecture of the Enterprise's "neck", it is unclear how large these inspection rooms are, and the room just up one level on M deck must reside between two torpedo tubes, and therefore be pretty small and tight. So it's safe to assume they aren't as large as the inspection rooms on N, O, P, Q, R, S, and T decks. Anyway, perhaps one day I'll get to model out the decks of the "Neck" of the Enterprise and will be able to visualize those intermix inspection rooms.
But until then, I've got to cap off the intermix shaft at the ceiling of N deck, which is convenient since it's also the uppermost level of the secondary hull. I could continue it up through the "neck" decks, but alas, I don't have the bandwidth to flesh out those decks at this time, so this "cap" will suffice for now.
I wanted to go with something that gives the impression that the intermix shaft continues upward, but closes the room off so I don't have to render everything above it. Here's a quick mockup of what I'm thinking. I don't want anything too flashy or distracting, as the primary focus of the engine room should ALWAYS be the intermix shaft itself.
I'm going to add some radial ribbing on the conic section there tomorrow night after sleeping on this idea, if I'm still feeling good about it then. What do you guys think?
Btw, here's an view of the entire secondary hull's engineering complex:
