I ordered this for my daughter's twin size mattress. It fits perfectly and seems to be good quality. We haven't tested the waterproof factor yet, but we are happy with it so far.
100% Waterproof Mattress Protector Cover with Dupont DuraShield Breathable Cotton Rich to Protect Mattress from Stains Spills Dust Mites Allergens. Fit Up to 21 Inch Mattress. (Twin Size): https://www.amazon.com/Waterproof-Protector-Durashield-Breathable-Allergens/dp/B07BPYKN7B