1/8" is fine. Even 1/16" is good. Just enough not to touch and allow the lid to vibrate. less wood? Eh very debatable. For a CBG its not as important in my opinion. IF you wanted to, punch a series of 1/2" holes through the neck sides (in the box of course) and that might provide some acoustics. Might also weaken the neck but then again, the neck isn't under extreme tension like a 6 or 12 string.
Next time I may make the sound recess even bigger, the neck is a full inch thick and dose not flex under string tension at all, I think the less wood in the box the better, at least that is what I have read.
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