Looking over a few forums, it appears that the shallow and deep-cups for the old BG Neo are just glued on with silicon adhesive.
Given that information, I had a couple ideas for how to attach custom 3d printed options..
The simplest idea would be to desing a cup similar to the Standard BG neo design, and use a similar method of attachment via silicon adhesive.
Alternatively, you could have the cup attached to the driver via the mounting holes of the Neo3, with a gasket between them for a proper seal.
Luckily this option would allow for attachment of the cup, without having to make it a more permanent addition, and make alignment much easier/consistent.
And by the same token, a front mounting bracket can be formed, with future options for flushmounts, wave-guides, or possibly even horns?
With a modular system like this you could even do some weird/experimental stuff, like porting the rear of the neo3 directly out the back of the cabinet. Even upgrades might be possible, if a "better" rear cup design is found...
I definitely want to sketch out some of these ideas, so any information on the rough dimensions of the GR/BG Neo3 & it's back cup so i can be a little
more accurate with my sketches.
And feel free to offer any ideas as well!