There are many things you can do, I'll name a few:

You can keep it in your own packaging (perhaps
some of the packaging it came in) and on a shelf
with no pressure on top.

You could buy an even bigger portfolio from one of
the many online artshops.  I think even the
popular Itoya portfolios come in 14x17.

You could put it in a single large presentation
case in a darkened closet.

I'm sure there are more things you can do too! ^_^

Hope this helps
Keys