Changes

An other possibility is to make a sign out of two small white boards that children use at school. Just join them with enough string so that you can fold those boards like you would close a book. The benefits compared to a single white board are that this sign takes a smaller place when folded (so it fits better in your bag), and that the text written doesn't rub you can write your destination in advance without it rubs out when the sign is in your bag.

Buy a pad of plain A4 paper, and use it inside a waterproof, dust-proof transparent plastic folder that you can find in most stationary shops. When writing, insert something under the page to stop the pen ink (or India ink) running through to the page underneath (a document bag is good for this). With a pad of paper, you can quickly flip through pages to find the sign you need. Also, the pad can be a great souvenir of your journey.