If those box frames aren't UV protective, I'd 
look into ones that are. Also, you said that 
they're plastic. Not all plastics are acid-free, 
another thing that's recommended when framing 
cels.

As for the light issue. . . I have two cels 
framed on my wall. *No* natural light hits them. 
They are both behind UV protective glass. The one 
of Clary is fine. The one of Keith has begun to 
line fade. . . I've since learned that Psychic 
Force cels are *very* prone to line-fading in a 
way that other cels don't seem to be. For the 
cels you have framed, you might want to find 
other fans of those shows - ask them what their 
experience has been with line fading in that 
show's cels. Another thing I did was (before I 
framed them) take a very up close, high DPI scan 
of the two cels I have framed. Now and again, I 
compare the cel to the scan to better keep track 
of how the cel is doing.

Many Sharp Smiles,
--Drac