As I posted on the introduction forum, what I do is reburbish, and occasionally customize WW-2 Camillus & KA BAR USMC knives that I buy in poor condition. Which of course have stacked leather handles. On some of the customized ones, I like to add spacers in the handles to jazz up the looks a bit. I make my spacers out common plastic items. Such as the bottoms of 5 gallon buckets. Or even oil bottles for real thin ones. So far they seem to work OK. Wondering what cheap common material you might use.