(By the way, that cel you got is from the Mihoshi 
Special.)

Be prepared to invest in significantly more 
expensive storage mediums than the 14" x 17" 
Itoyas.

I have a 18" x 24" portfolio to keep my > 17" pan 
cels in.  It's a pretty nice zippered case, has 
10 archival sleeves (for up to 20 cels), and a 
handle for easy hanging.  This is where I got 
mine at:

http://www.utrechtart.com/dsp_view_product.cfm?
classID=2210&subclassID=221011&brandname=Prat%
2FEuropa%20Inc%2E&Item=24615

However, I do have one pan cel which is even too 
big for that (~27").  For the extremely large 
pans, or if you don't want to get a portfolio and 
want a storage solution for only 1 pan cel (maybe 
2), you should probably get an archival print 
protector like these:

http://www.utrechtart.com/dsp_view_products.cfm?
classID=2210&subclassID=221010&brandname=Angler

The particular size I want is on backorder, so I 
just keep my 27" pan in a case of sorts made from 
priority mail box cardboard and kept under my bed.

I like pans since they tend to show a lot of 
detail, especially for full character shots.