HDPE is usually the default for me when it comes to Featherweights. I've used 10mm in the past which held up remarkably well for what it was on The Honey Badger but I tend to use 15mm+ these days for anything structural. Azriel (my current FW), for example, is 15mm HDPE with a 6mm polycarbonate base and lid from left over parts from previous builds. The axe carrier is 20mm HDPE.