Your situation mirrors my experience with cloth diapers at night. They need to be fastened above the hips to ensure a comfortable and effective fit. This places the top rear portion of the fabric well above the waist. Waterproof covers, PUL or plastic may not come up high enough to be effective. The other problem with any cover is that it can work its way downward during the night as you move around. PUL and traditional plastic pants (with fabric covered elastic) place an absorbent and leak pathway just where you need protection. The covered elastic PUL pants may work but my experience is that they weep and wick at the gussets (elastic in the legs and waist) because of the way they are sewn.
Your best option could be the Gary urethane plastic pants with the wrapped elastics in a larger than normal size. They are not cheap but they do have a much longer life span than regular plastic pants. One possible source is
www.myprotex.com. Look for the overnight collectors edition pant. I question the product name but the website shows this as being manufactured by Gary Mfg.