If you have any access to a Photography 
studio (even a portrait place in the mall) ask 
them for one of the nifty cardboard film boxes.  
They are super sturdy and are big enough to 
keep even super size cels and backgrounds 
flat.  Plus they are such thick cardboard that 
they don't let in much moisture and very hard 
to damage (I've moved twice and not a cel 
casualty).