For most of my cels (except the 
oversized/background cels), I just replace the 
bag with a 11x14 polypropylene envelope open on 
both ends.  These envelopes perfectly fit in 
11x14 Itoya books.

Most often, I have to do that since the bag the 
cel originally came is usually warped (and/or 
made outta polyethylene [you can definitely tell 
if it's polyethylene if the bag seems unusually 
crappy]).  Once, I had a cel come in a zip lock 
bag.  (It perfectly fit standard sized cels, 
though, heh.)