You can pack the dovetail and rings separately. They are completely replaceable.
The entire lens cell can come off. The Astro-physics GT130 is actually designed to travel this way.
They provide a nice lid for the rear of the lens cell. But you can make one. Take a piece of good quality cardboard and make a long 1 inch strip an half an inch or so longer than your OTA. Coat the strip with a thick layer of duct tape. Bend it around and join it to make a circle. Now take a nice piece of box cardboard (thicker and with a bit of substance to it) and cut out a circle equal to the size of your objective, and coat it in several layers of duct tape. Then, using more duct tape, join it to the circular strip. You will find that as you've put maybe half a dozen layers down you're getting a very strong cover with cushion type effect. More than enough to protect the open side of the refractor. You can make one for the OTA too if you want to.
You might even make if for both sides of the tube so you can remove the focuser.
The FS128 is a big long scope. However, I was able to get the focuser and the lens cell into a Lowes tool box and then put the whole tool box into another box for shipping. Or you could carry focuser and optics on to a plane. Your tube might be small enough to be packed with these other two components. Notice that the focuser is wrapped in closed cell foam (sleeping bag pad) so that it won't damage the lens cell. The hard metal cover is facing the focuser. There is a cap on the other side that I fashioned for this shipping.
The tube can at your choice be packed separately or carried on board if you can contrive it--either in the same package with the rest of next to them.
Because, believe me, the tube is A LOT shorter when you remove the lens cell & dew shield on one end and the focuser on the other.
Everything reassembles pretty easily. The focuser end has a flange that takes the focuser the flange stays on the tube nothing to worry about there. The lens cell screws on to the tube. Astro-physics followed this same basic idea but they made it more elegant and put it all in a Pelican case, which is what I would recommend for you. I would not let the luggage people have the refractor but I would consider shipping it for pickup to my destination with adequate insurance. A scope in a Pelican case could go in the overhead. It would be protected against the usual process of stuffing things in. I mention it because "under the seat" is a tough criterion.
I haven't really inquired but in the worst case situaton, if you checked your tube and it did not arrive or the whole suitcase got lost, I think most OEMs would be able to replace your tube. It's the optics that are the most precious element. However, some tubes are more nicely manufactured than others so prices may vary. When I inquired with TNR their FS128 tubes were cheap, as in, if I remember, less than $150.
I don't think that would be true of CFF or Astro-Physics. I think their tubes would cost more.
a) design it with the dew shield extended, which would allow me to leave the mounting rings/rail in position to be readily balanced on my mount? This'll increase storage space required and cost/weight. (scope is 20lbs, extended it would make the case about 48"
b) design it with the dew shield retracted, which would make the case much shorter, make the case less expensive and lighter, but would require the mounting rings/rail to be slid back. This means I'd have to make the adjustments each time I balance the scope on my mount. I did a couple "practice runs" and it did not take much to reposition the mounting rings/rail. (retracted would make the case about 40")