Shield flexibility

[maximilian]

So, how flexible may a shield be, and why would it be limited? I ask because I am considering a plexiglass core for a strap shield, and the one I have is rather flexible. It is probably also tougher and more durable than the cores I have seen discussed before. Are there specific guidelines or limits to shield floppiness? The core I am in possession of is a paintball shield, similar to a riot shield.