Cold finger design based off of one done by Anat here:
-Link to Anats Hyper Mod
Much more detailed instructions on how to disassemble the Rebel XSi/450D are here:
-Gary Honis's instructions
Goal:
To cool the imaging sensor of the camera as much as possible to reduce noise signal of the camera at high ISO (sensitivity) over long exposures.
The aluminum "Cold Finger" sits under the imaging chip circuit board, and above the sensor itself. There is a very small gap here where the finger was inserted. It is then sealed up with hot glue all around to prevent moisture buildup. The top of the exposed circuit board is also sealed up with hot glue to prevent moisture. Feel free to leave a comment.

Shot of the sensor with the cold finger inserted. The electrical tape is a precaution to prevent a short on the circuit board.

Shot of the sensor with hot glue sealing up the circuit board to prevent moisture buildup.

I have removed the filters to make the sensor pick up infrared light.

Shot with the sensor reinserted.

Reassembling camera.

Camera reassembled.

Infrared test image. Surprisingly the auto focus worked rather well still. Not supposed to work when you remove the sensor filters.

3 minute exposure at ISO 1600 not cooled.

3 minute exposure ISO 1600 cooled.

Shot of the final assembly. Not very pretty but works good.