The suggestion has been made (on the rec.arts.startrek.tech newsgroup) to retrofit the Galaxy's Captain's Yacht or the Intrepid's Aerowing shuttle into a heavilly refitted Excelsior hull. While they cannot be put in the center of the primary hull underside, (the sensor dome is there) an Aerowing would fit forward between the sensor dome and the underside cutout.

(Actually that's not an Aerowing, I just attached a plank across the bottom of a Runabout.)

The one problem is that it cannot be put there without losing a forward phaser bank. However, two can be placed port/starboard of the center, between the side phaser banks.

The curve of the underside creates an unfortunate lack of symetry in the Aerowing as one side is further in the hull than the other. The final view shows a front view of the Excelsior saucer (without a bridge module). You can see three of the phaser banks and one nacell of each Aerowing peeking out from underneath. The bottom two objects are front and underside views of a Galaxy's Captains Yacht for size comparissons. (The shot of Runabouts in formation with the Galaxy class USS Odyssey from the DS9 episode "The Jem'Hadar" are a good scale reference.)