i dont think u should screw it or glue it .ive made a couple with a petbowl. leave it float . when u stick it on withglue or screws it wil only sit there looking pretty as it wont be able to "resonate" ...
I've got one in the works and the plan is to let it float. But, like others have said, it just depends on whether or not you're happy with it.
Also, the style of bowl may dictate your options.
I get my Cat & Dog Bowls from 99 Cent Only Store. They are not only cheaper, but thinner stainless steel. I always use the bottom of a Pardon flip top box & the only brace is the 4 blocks for the screws to hold the bowl down. I use a neck held on with a hanger bolt. I have made all sizes from 15 inch concert uke, to 25 inch 3 & 4 stringers, most with rod peizo's and preamps. I love the tinny sound.