With no regards to the golden rule, maybe something like;
23.5"x18.5"x46.5" inside dimensions = 11.70 cu. ft minus the volume of the horn if you want to mount it inside the box.
23.5"x18.5"x40.5" inside dimensions = 10.19 cu. ft if you wanted to put the horn on top of the box.
I think that the front and back panels should use a 1/4" dado into the sides, top, and bottom panels as well as all of those panels to each other.








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