HDPE is great for a wasteboard (and I’ve been using one for a long time, pic below), but if by “spoilboard” you mean the MDF plate that came with the machine and attaches to the structure of the Shapeoko (I call it the “baseboard”), then I don’t think you should use HDPE for that:
- HDPE is soft and unless you go for a very thick plate, it’s not going to be any stiffer than MDF.
- it’s expensive, so the price of the (thick) HDPE plate required to cover the full machine footprint would sting.
Unless you really need to optimize your Z travel, and considering you have a setup that works great for you, I would add a supplementary HDPE wasteboard on the top of the existing one. It does not have to be thick then, so it will be less expensive too.