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This is an 8x8x8 using 1/2″ material for a total internal volume of .198 cubic feet. If you wanted to use 3/4″ material, you can make it an 8.5×8.5×8.5 . That will keep the internal cubic volume the same. If you want to go a little bigger, you should be fine, but it will hit the mechanical limits of the passive radiator sooner. Not by much, but just something to think about.