Have you considered using LED rope lighting, which creates far less heat than incandescent rope lighting?
If not, I'm a little worried for you guys, after reading some of the warnings on http://www.centsibleholidaylighting....leshooting.htm
I'm pretty sure I remember reading a "DIY backlit movie poster box" thread that used incandescent rope lighting, and had a picture of melted rope lighting found on the inside after heavy usage. I wonder if anyone can find that thread and photo I'm referencing...
Anyway, make sure you monitor the temperature of your DIY lightbox, and don't leave it on unattended.
Anyone know of a good source for LED rope lighting that is competitively priced with incandescent rope lighting?