For the stack I'm using 3mm ply cut out into 90mm discs. I then cut out a circle in the middle of each disc (there will is 5 discs in total) to enable a piece of 32mm PVC tubing can pass through it.This tubing is where the tube from my Steam Generator Unit will be positioned.

The outer of the stack is made using some strip-wood which I then sand & filled followed by some high build primer ready for the spraying.