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First of all, welcome to the forum, Southern somker! As for your question, the opening under your plate needs to be as big if not slightly larger then the opening from the firebox to cook chamber (mine is the same all the way down). This also goes for the end of the RF plate, opposite of the firebox, theis opening needs to be at least as big as the firebox to chamber opening, preferably bigger. This will keep from restricting any air flow, helping with draft.
This is a pic of my firebox prior to attaching the RF plate to the top of the firebox inside the cook chamber. The opening you see is the same all the way to the end of the RF plate.