You can get 20L roll top dry bags on Amazon for less than $20/bag and tie them onto the rack. I did this for a whole tour. just make sure you find a way to secure them to your rack so they don't go into the wheel.

https://www.amazon.com/Floating-Waterproof-Kayaking-Swimming-Snowboarding/dp/B01LX0AOPX/ref=sr_1_5?dchild=1&keywords=20l%2Bdry%2Bbag&qid=1601697766&sr=8-5&th=1&psc=1