The spacers between the cel and the background 
will probably do their job; the spacers will have 
more of an effect keeping the cel away from the 
background than the amount of force the cel will 
exert in trying to bend inward to the background.

The only thing I can think of is--- did you use 
archival tape, or regular ol' Scotch tape? :o) 
People can sometimes be very careful about acid 
free matting, UV-protective glass, and the whole 
nine yards, but can often forget that archival-
quality tape is important, too. Anyone who has 
old game boxes with split sides that they've 
tried to tape together knows what they look like 
after a few years... and often times, the tape 
doesn't hold completely. Even so, just the 
adhesive might discolor the part of the acetate 
that it touches. I don't know; I haven't 
experimented too much with archival tape on 
plastic.

If you're happy with it, that's terrific. If not, 
you might want to try some of those sticky-
adhesive photo corner mounts. You might arrange 
them on the back of your mat so that the cel can 
slip inside it. That way, it's completely 
reversible.

It looks terrific! :o)

-Cres