If you want a tight fitting lid on a wooden bowl, I think a custom woodworker might be able to design you something along the lines of a rubber 0-ring set into the bottom half, with a matching notch on the lid. That would be about as tight as you could get it, I think.
On the other hand, most soap bowls are designed to allow a little air circulation so the soap doesn't get mushy. This is true whether the bowl is made of wood, ceramic, glass, or plastic.
About the only soap bowl I've ever seen with a snug fitting lid is L'Occitane Cade, which is made of metal (Tin? Aluminum? Pewter?). I've never owned one, but I have seen it in stores, and the cover looks like it would stay on if you turned it upside down.
Gotta ask, though ... why do you want the screw on lid? What purpose would it serve that the looser fitting lids do not?
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