We have 2 different models on our Special Design 2x12" Extension speaker cabinets.
Both designs use 15mm (5/8") Baltic Birch plywood on the shells and 12mm (1/2") on the front and back panels.
The Open Back is 27.75"Wx20.25"Hx11"D
The Closed back is 32"Wx20.5"Hx11.5"D
The closed back has a center baffle/support secured with 2 machine screws (Not wood or sheet metal type screws) into metal nutserts rather than wood screws going directly into the wood, which eventually strip out. The closed back has the V Front design option only.