Reccomended Custom BG Media?

Hello...Recently bought a fairly expensive cel, and I was going to have a friend (very talented artist) make a reproduction-ish background for it. I'm getting it professionally framed; however, what media is safe to use with cel backgrounds (eg, chemical problems, dust etc)? By media I mean colored pencil, water-color, oil, acrylic, etc. From what I have seen you always have matting between the cel and BG anyway, but I'd rather be safe than sorry.

Thanks in advance; also, the galleries here are so pretty :)