Feline incontinence ideas?

My elderly (almost 20) cat Mitts has had a couple of "accidents" recently. This is not a big problem when he lets go on a hard-surfaced floor, but is when it happens at night in my/our bed! I have tried to put waterproof underpads
on the area where he usually sleeps, but he moves around.

I have tried to use techniques from when my children were toddlers (taking him to the litter box when I notice he has not been recently) and using the phrase "Go Box" to associate him with doing so. He's a pretty smart cat, and when I told him "Go Box" when he finished eating the other day, he squatted immediately, even though he was in the kitchen and not in the litter box. At least he has associated meaning with the phrase, if not exactly what I intended.

Has anyone tried diapering a cat? He sleeps a lot but is still mobile despite arthritis, so I don't think he would take well to such a garment. Any other ideas? He is sweet and alert, so I don't want to shorten his life for my convenience.