Go and get yourself some thicker material, use the template you have to transfer it in to the new material and just think of it as good practice handeling a router. Shims in this case are NOT a good idea, as you have said if they slip, your buggerd. even though you will have it clamped down well things can slip from time to time. Yeh, if you get some 3/4" material you should be able to work with that.