I usually cut a piece of an old black T-Shirt and use a staplegun to cover the hole from the inside of the cabinet (inside the marquee area). Then I install the speaker over it, sandwiching the cloth to the wood. Finally, to cover any imperfections of light leaking anywhere else I use Weatherseal tape, the kind you would use on edges of a doorway or window. They vary in size and are great for blocking light, especially on the edges of the marquee. You simply use the self-adhesive tape to line the edges from the inside.
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