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The Cougar Case is a pre-fabricated case made in Australia.
Paul's original design used a flat aluminum base plate, U-shaped aluminum cover and plastic end plates.
The base plate was 8"x11"x3/8" aluminum. The material and thickness were chosen for heat dissipation rather than strength.
The sides and top were bent from a 16"x11"x1/8" piece of aluminum. The enclosed volume was approximately 11" x 6-3/4" x 4", with 1/2" outward-facing mounting wings.
An aluminum base plate was chosen primarily for its good thermal conductivity. It's much less expensive and lighter than the few alternate choices (e.g. copper or silver). Using aluminum for the upper case avoided dissimilar metallic joints, and provided some additional cooling surface.
Using conductive material for most of the case blocks some of electrical noise. Aluminum does an excellent job of blocking EMI, although this advantage is greatly reduced with a non-metallic end piece.
Internally the active power devices (e.g. MOSFETs and diodes) are thermally connected but electrically isolated from a heat spreader bar. The first few units used a 0.75"x1.5"x8" copper heat spreader, while later units used aluminum bars with the same dimensions.
The heat spreader was stacked with the main power board and bolted to the base. Four fasteners were used, with with 2.5" long 1/4" screws covered with heat shrink going through 5/16" holes in the heat spreader and circuit board.