Plastic/Glass for a very large, custom frame
A clear Mylar sheet could work.

Clear, moisture resistant, acid free and archival.
And available in the size and price range you are looking for.

Here's an example available on Amazon 
https://www.amazon.com/gp/aw/d/B0027A3HKG/ref=mp_s_a_1_1?ie=UTF8&amp;qid=1473127673&amp;sr=8-1&amp;pi=SY200_QL40&amp;keywords=clear+mylar+roll&amp;dpPl=1&amp;dpID=31N50Emps-L&amp;ref=plSrch