I only use two cel books right now.  The standard 
11" x 14" Itoya portfolio for my non BG cels and 
the 14" x 17" Itoya portfolio for my BG cels.  
However, I have 4 cels that are longer than 17".  
(My biggest cel is 14" x 22".)

For these 17 " cels, I made custom cel bags by 
cutting apart and taping together several 
sheets/envelopes of polypropyelene (mylar would 
work better but is more expensive; cutting apart 
extra cel bags would also work).  Since I don't 
have a portfolio large enough for these cels, I 
made storage cases for them out of priority mail 
boxes (which are free) so that I can store the 
cels vertically in my closet next to my cel books.

Every time I get enough money to purchase an 
extra large portfolio, an expensive cel just 
happens to show up, heh.  Later.