Thanks for the input, and I had some of the same ideas...maybe fastening two 3mm panels together (Val, I already checked with Sun and they do stock 10x5 sheets of 3mm). Whatever is up there now is probably 1/2" thick and there are metal tabs around the frame creating a channel to hold the panel in place, so whatever I end up using has to fit snugly in the channel. (see this close up image)
Gino - I already quoted running two lift rentals on this job: one to go pull the old panel down and measure, and another to install a new one. The structure itself is over 50 yrs old, and I'm guessing the current
sign face is at least 20-25 yrs old based on the looks of it.