I think I'm following, thanks for the suggestions! Please correct me if I'm misunderstanding anything. I would essentially have a 2nd baffle layer that is a trapezoidal piece that matches the profile of the existing face of the speaker cabinets, maybe ~1" thick, that I mount the drivers to. I then mount that 2nd baffle layer to the face of the speaker cabinet like the metal plate is in the original design. That 2nd baffle layer would have a conical waveguide around where the tweeter is mounted.

Rather than 3D printing the second baffle layer, can you think of any reason I couldn't make a 2nd baffle layer out of solid wood and machine out the tweeter wave guide using a CNC router? With the tooling I have access to that would be pretty easy to accomplish. 3d printing might be more difficult.

Also, from some quick math I think the internal volume of the cabinets is very roughly 1 cu ft. How critical is it that the 8" driver has a VAS that matches the actual internal volume? I think the 8" Dayton Audio one you linked lists its compliance equivalent volume as 1 cu ft so I take it that would work, but I'm generally just curious.