Hi Mike
Just one or two ideas, Think about hanging the signs from the post rail with chains, top and bottom of the sign, this will center them in a frame, plus they will not blow away.
Dog collar chain works great for this type of low hang job,it's plated and will not rust,it's small and strong,hard to see and hard for some one to walk off with your new sign...you know kids (big and small ones) I'm sure some of them would like the sign in the bedroom/basement at home, hahahahahaha
I have had some get legs over night and just walk off when they where down low to the ground,most people don't have the tools to remove the chain or bolts with them.
Plus the white paint on the rail will stay white.
Chain for hanging lights can be removed with just a little turn as you know, I'm sure.
Just a idea.
Bj