Well, I used to have a cel that was about 3 feet 
in length, and this is what I did to store it.  I 
went and bought a huge wardrobe box at the 
packing store, like those boxes you hang nice 
clothes in when you're moving.  I cut the back of 
it off to use as a backer for the cel.  Then, I 
took two standard sized envelopes and cut them in 
half.  You use those as corners to hold the cel 
together, and then you tape the envelopes to the 
cardboard backing.  I went to Hobby Lobby and 
bought some acetate there and neatly wraped the 
whole thing with it.  You get a very compact and 
nice looking display that is also safe for the 
cel.  You can also use a large sheet of cold 
press board, or matte board and choose a nice 
color for the backing.  If you want to get fancy 
with the corners you can also use construction 
paper or other stuff for those instead of 
envelopes.  I just picked the easiest and 
cheapest rout.  :)  This way you can store the 
cel upright to where people can see it, and no 
dust or anything can mar the cel.  It also stays 
nice and flat, and the corners are protected. 

 As for bringing them to a con like that, I flew 
on an airline that had a "portfolio" storage 
area, in the back of the plane.  It was really a 
thin space behind the last seat on the plane that 
was nice for storing large flat items.  I had to 
carry a big mounted poster back from a trip once 
like that.  Im sure you could just ask the flight 
attendant to help you store the cel while keeping 
it flat.  Good luck!  This is why I dont like big 
cels that much...too hard to deal with them most 
of the time.