Jim, youre going to like this one. i use a copper shim from CPUfx LOL!! (dont worry, i got it for free ;p)
fixittt makes a valid point; ALWAYS check the shim to ensure its not too tall or too short. i used the edge of a credit card (or business card; anything flat will work) to make sure it was the proper thickness. you lay the shim on the CPU and then test to make sure the shim and core are the same hieght; no light should be comming from under the core. you should probably get a non-conductive shim, as ive heard of morons toasting their chips cuz a copper shim shorted out the bridges or some other such junk (ive mounted my CAK several times and have had absolutely no issues whatsoever)
here are Tweakmonster's aluminum shims.