Abstract: A plurality of like modules having an open sided C-frame construction are provided with slot-forming guides for receiving a plurality of circuit boards which are inserted from the open side. The circuit boards, when fully received in the guide slot, are engageable with electrical connectorings on the main body section of the C-frame. A plenum chamber is formed on the bottommost surface of the plurality of C-frame modules, and vent slots are provided through the modules at the upper and lower limbs of the C-frame, thereby permitting air to be ducted through the plenum chamber and convected up through the vent slots, thereby dissipating heat from the circuit boards. Means is provided for connecting the plurality of modules in a predetermined fashion to one another, to form a rigid assembly.