I’ve recently discovered the utility of hardboard in making jigs for CNC routers, so thank you all for the info, it’s extremely timely!
(Attach workpiece to hardboard, use plastic brad nails to attach hardboard to spoilboard. Gives you cheap sacrificial layer that’s easily manipulated, not too thick, won’t harm any cutter and is extremely flat. You can strip the hardboard by using a chisel to shear off the brad nails, leaving you with almost no damage to your spoilboard.)