Our high school ag class has cut a 2-sided sign out of metal with their plasma torch and need a colored "insert" to go in between the panels so it is readable from both sides (the sign is two sheets of metal with letters cut out and then welded to each other back to back with a gap between them) It's an outdoor sign and the slot for the insert is 1/2" - 3/4" thick. What would be the best material to use?