GORILLA GRIP Washable Underpads, Waterproof Reusable Heavy Absorbency Incontinence Bed Pads, Leak Proof Toddler Mattress Pad, 52x34 Slip Resistant Protector for Bedwetting, Adults and Kids, White

They make these pads for “bed wetters” that you can just throw down on top of your sheets. Some of them have grippers on the back so they’ll stay in place. Like this one from Amazon. I’d recommend the 52x34 or the 76x34. They hold a LOT of liquid.