2 grommeted holes with blowoff tubes in each. The one to the bottom of the union, where the spillover goes back to the bucket is inserted below the top of the wort level, The other hovers above the wort like a normal blowoff tube and when the fermentation explodes it will push out through that hose into the top of your collection vessel. And you don't want too big of a fermenter because the blowoff is the whole point of doing this.
BYO had an article on how to build it.
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