The background is in the bag to prevent it from 
sticking to the cel.  

If you frame the two, try keeping the cel and bg 
from pressing against each other. If they lightly 
touch, they probably won't stick. To be 
absolutely sure, place a sheet of polypropylene 
between the two. 

A good quality polypropylene sheet shouldn't 
wrinkle much, at least not the parts that can be 
seen.