Changes

Make a sign of a large sheet of brightly colored paper glued to a sheet of cardboard. Cover this with self adhesive transparent book wrap. Now you made yourself have a re-usable sign wich which you can use with a whiteboard marker and clean again with a small towel.

Make use of waterproof, dustproof dust-proof document bags. There you can store many sheets of paper and signs you can reuse. Nothing falls out, you could hang them somewhere and you can use both sides of the folder to have two signs at once. Ortlieb is one company that produces good ones.

Buy a pad of A4 plain paper, and use it inside a waterproof, dustproof 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 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.