I use the brinsea octogon 20. I love it. Hatch rate is 90%. I go with 30-40% humidity until lockdown then bump to 60%+. I use the cool down feature set for 30 minutes every 24 hours. It holds temps and humidity very steady! I thought about getting the humidty pump but I only need to add water...
Arsenic is no longer used in PT lumber. A copper solution is used to deter insects and fungi. I still wouldn't use it for compost bin walls. Maybe use pallets?? Easy to find, usually free!