My only concern with using a piece of copper with thermal paste on the sides is that the thermal conductivity of copper may not be the same as the original thermal pad. I would assume that it could work, but I would do my best to try and replace the pad with a similar one.

A lot of time, there will be people selling pre-cut slices of thermal pad on ebay.

I have also purchased thermal pads from Digi-Key.

You should be able to read the processor temperature on the desktop. If you do decide to try and use a copper spacer, then be sure to check the temperature and make sure that it is within operating limits.

Good luck and let us know how it works out.

Ok. Thanks for the answers. I tried the copper alternative and it worked! The first thing was: made a clay mold to know the proper thickness of the gap; then I got a piece of copper (wire conductor with 4 milimeter in diameter), cut a piece 2 centimeter long and start to smashing it until got about the right thickness... Then I cut the piece so I got a piece of (1 x 1.5) square centimeter and I did the first presentation into the space to be filled; From here on out it was a matter of polishing (with a bench grinder) the piece until achieve the proper fit. Put very thin layer of artic silver 5 on chipset, heatsink, and the both sides of the copper piece, joined and then separated them to check de paste distribution and the paste excess areas, with a card smoothed the past and joined again; I made sure about the firmness of the piece and turn on the laptop. After 20 minutes of operation (playing DVD movie and watching manga from internet) check Everest panel and it gave me 40ºC for processor, 52ºC for motherboard and 57ºC for chipset; then measured temperatures with a thermocouple and the chipset gave me 42ºC on the body of the chip and 48ºC on the top of the chip (very near to the black area where the piece of copper makes contact); for processor it wasn't possible measure with thermocouple 'cause the heatsink completely covers the processor... until this moment the fan was switched on only twice at a slow speed not like before: at top speed.

So the conclusion is: the piece of copper works but only if you can fit it properly to fill accurately the gap between the chip and the heatsink.

Thermal paste