That happens to us a ton. A fellow 1191 owner complained to me about the same thing WRT the sink. I bought rubber corks that I jam in there (sink and shower) as tight as I can and it sometimes works, sometimes doesn't. When it doesn't it's ugly. I've had the walls of the bathroom covered with the P-trap water many times. I try to remember to lay a towel in the sink on long trips to at least keep the splashing down. I'd like to find some corks that are adjustable....seems like I saw some once where there is a flat washer on the bottom and on the top, and you crank a bolt to squash the rubber out wide enough to lock into place.
Anybody seen these anywhere?
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You might try McMaster Carr. I needed some variuos size plugs similar to what you have described, some with wing nuts , some that had to be inflated with air. Can't remember what they are called to make the search easier.
Brian