Use a tube for poster. I know many people will go 
crazy about that for absolutly no reason. But 
still, if the tube is large enough (around 4 inch 
in diameter or more), there is absolutly no risk 
to the cel even if it has a background. (If it has 
a background try to get a larger tube if at all 
possible).

Cheers,
Yann Stettler