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For a quick fit with no fiddling with foam pieces (1/2 inch makes a huge difference in fit).
You really do want to have something there or you will be very uncomfortable paddling it; and it will actually be dangerous in terms of injury to your feet if you hit something or when you go vertical repeatedly if you don't have anything in it.
However, based on your question, why are you looking at the Fuse? Unless you specifically want a very short and playful river runner (which is actually what a lot of people do want, nothing wrong with this), you probably want something else...