Hi there ^_^x

To answer your question, not all plastics are 
created equal. Many plastic bags contain acids 
that can eat away at you cels if you store them 
in it. It is important that the bags be archival 
safe, that means acid free. Also the box you have 
should also be acid free, you can check with the 
manufacturer if it is, there are a lot of 
companies that make boxes to preserve items that 
will fit your cels. 

Ray