Jon,
I'm back from the land of Delta and thinking that you might want to look at the Kittredge style again. 36" OD is plenty of room for one person and you could then have a 2-position sub, upright or down in the hull. That means two viewports, with more to come later. George was pretty generous with internal framing and if the Nektons are anything to go by (and they are) then you don't need that much. All the Nektons have two, count 'em, two internal ring stiffeners, one in front of and one behind the conning tower. 1/2" wall thickness gets A, B & C to 1000' with test depths of 1500' and 9/16" gets Delta to 1200 and 1700 respectively. The nice thing is that you can build it nicely streamlined and be able to tow the thing around. Plus there will be room for junk and stuff and bits and pieces. I probably wouldn't make it any smaller than 36" but you don't have to make it any bigger, either. Depends on what you see as your overall weight requirement.