Start by pinning the sign to two A-frames via the middle screw hole. Align the A-frames and get the other twelve screws in, flushing the sign edge to the frame as you go. Use 1-1/4" screws with a #10 washer for superior old, as the aluminum is so springy it can snap and pull out screws if it doesn't feel like cooperating.
Pop in a cross-bar with two long screws per side and some glue.
Take a rounded bastard file and hit each of the holes along the seams so they won't scratch or snag clothing. A couple quick strokes per will do. Also make sure to sand or file the coating on the sign where it cracks; the sharp plastic there is actually more of a scratch hazard than the aluminum itself. Older painted signs don't have this problem.
Clean them off with some dish soap and a sponge, then a final swipe with Windex and have yourself a well-deserved seat.