iBook G4 logic board "fix"

Some may remember my thread a while back about the iBook G4 I purchased on eBay which was having freezing issues. So far, nothing I've tried has worked. I figure trying this can't hurt anything if I'm careful enough (wish me luck. )

The solution I am talking about is placing a shim between the logic board and the case, and my question is this; what exactly is a shim? I understand the concept, but I am clueless as to what to actually use for one. A thin piece of metal? Plastic? Fabric? What kind of material exactly works best for this purpose?

 

Thank you very much, I just wanted to make sure there wasn't anything specific I needed to use. I shall attempt this very soon, and update with results.