There's a dingus called a "luggable loo", that's a toilet seat designed to snap onto the top of a 5-gallon bucket. As a "pointer", I don't have a problem peeing in a bucket. A "setter", though, is another story. And sometimes "pee" ain't what you need to do. A cheaper option, maybe, would be to get 4 L-brackets and screw them to the bottom of a toilet seat, so when you placed it on top of a 5-gallon bucket it couldn't slide off.

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I always take precautions