Ok, so when I brought up the undercut, I was talking about the
secondary hull undercut, and that's why I mentioned the Excelsior's being so very long. The Excelsior does not have much of an undercut on the saucer, except for a ring around its middle. I was trying to suggest the reason for the secondary hull undercut might be that the shuttle facilities are designed as sort of "tacked on" to the tube-shaped hull, and that, as a design like the Exclesior's was up-fitted, a future version might one day have less undercut, which is what the Ambassador's design suggests. In the case of the original poster's project, I was suggesting that as he did his layouts, he could look to the saucer and see if he wanted to include drop-points for travel pods, which he chose not to do. I was thinking that this would address how ships without apparent shuttle bays could deal with evacuation in a crisis.
Regarding the undercut of the
saucer, I'm not sure what purpose it could serve. I don't think that the huge rec deck makes sense being in the saucer, regardless of the design, because a lesser ship might not
need that kind of recreation facility. (When the show crew needs to assemble in
Voyager, they just go to one of the cargo bays; otherwise smaller recreation faculties suffice..) Putting rec deck in the secondary hull means that you instantly gain the extra space in a plug-and-play fashion along with the engine and shuttle facilities. That was not what was designer intent, though, apparently.
The only way that the rec deck being in the saucer makes much sense to me is that maybe this space is not so big or open on ships with no secondary hull. Perhaps it is smaller and part of it becomes cargo bay with access to turbolifts. Perhaps this is a travel-pod launching area for ships that do not have dedicated shuttle bay space. Reliant's starboard bay lines up with this general location, so it could line up with that.
Perhaps these ideas will inspire some ideas for modifying the space to fit on the ship and how it might be altered to appear as it might have for TOS. If the goal was to view both versions of the ship as being as much the same as possible, this might spark some design paths
