Photo by tookapic https://pixabay.com/en/users/tookapic-1386459/
When using a soft suitcase, bag, duffel or even back pack line it with a heavy duty garbage bag and place your clothes, paperwork etc. in that to protect them from rain and water damage
*Tried and tested on a recent Bali trip were the baggage sat on a trolley during a heavy down pour