I use the Kokuyo one and am pretty happy with it.

Or possibly just buying a cover for the notebook. I don't know about polypropylene, but Canvas is certainly a common option which surely can stand getting wet.

Options with notebook:

https://www.lochby.com/

https://smile.amazon.com/Kokuyo-notebook-systemic-corresponding-V685B-DB/dp/B00S0GQS5C/ref=sr_1_68?crid=2M0N6YM14LX1R&keywords=canvas+a5+journal+cover&qid=1650139110&sprefix=canvas+a5+journal+cover%2Caps%2C313&sr=8-68

These covers can easily replace the interior journal, they are a standard size (A5)