One night I was laying in bed hung over and all of a sudden I look over to see my adult normal getting out of his tub. So I put him back in his tub and (since I don't have a rack) I ended up going to Walmart and getting a vinyl strap and a clip, wrapped that around the tub and clipped it shut. He's not gotten out since.
Mind you when I had him in a tank he got out too, which originally turned me on to the idea of using the vinyl straps.