Hey Ken, instead of having a heatsink on the cold side why not move the TEC to the bottom of the box under the camera and use some copper plate to attach from the TEC to the tripod adapter? That way you are using the camera frame to act as your cooling plate right where you want it, inside your camera! Have done this with my other cameras and even without a box around the camera I usually get 7-8c cooling inside the camera. Should double that inside a box, even more. Have you checked Gary Honis website for his cooled 450D
He uses the aluminum box as cold plate more or less. The camera would also be attached to the aluminum by the tripod socket also.
http://ghonis2.ho8.com/rebelmod450d16c.html
cheers