We have our fair share on night time accidents with two young growing boys. I feel like we could never have enough waterproof mattress protectors. While going though my fabric stash, I found some terrycloth backed with waterproofing material. It was just enough for a small bed mat. I backed it with some leftover flannel from my fox PJ pants.
It's not one of the bigger mattress protectors we have, but it makes a great backup.