This is a 3D modelling of the V.B.S.S. subwoofer from AVSForum (https://www.avsforum.com/threads/the-v-b-s-s-diy-subwoofer-design-thread.2226642/) using 18mm Baltic Birch and slotted vent. No bracing or speaker reinfocement has been added, but would be needed on a finished cabinet.