I finally completed this project using HDPE for my slats. I saw that someone else had used phenolic material, but HDPE was considerably cheaper and it seems to have worked quite well. In another thread I detailed the process and included links to where I sourced the material.
If you don’t want to give up your slats I think HDPE is the perfect material for the HDM other than the challenge it presents machining it. It will allow me to switch between a MDF spoilboard or aluminum fixture plate pretty easily depending on what I want to machine while being immune to liquid coolant.
