I mark both ends of the slot and drill a small hole in each end. There's a skinny blade for a zona saw that you can then drop in the hole and connect them together.
You could also just drill a series of 1/16" holes along the length of the slot and then file it out. Once the sign goes on, you wont notice what you've done to the roof, but its important that you get it straight enough that the sign material slides through without flexing or bending. Those EL elements are a bit fragile at the connection tab.
