When working on old Fenders or any old bolt-on neck guitars, I've often come across cut up cigarette packets. I guess they're just about the correct width for some neck plates and I've even seen small cut-off strips from that strong plastic stuff you tie things to pallets with! But I think OldG was right to suggest wood or similar. There is a degree of trial and error involved and I'm often surprised at how much difference a tiny shim can make. Good Luck.