Making the case, gallon cans made of tin plate are hard to find, but I still have 2. The end & base are thicker from an old DVD player. Wedge the thin sides in position with wood, cut holes to fit over fire tubes fit the plate in place, draw around the case, cut out 10mm outside the line. Bend the 10mm edges inward drill and tap in place. Same with the other end using the bushes this time. After fitting take apart and enlarge the holes so the fire tubes & bushes don't touch the case. I will use soldering mat as the insulation.
The new fitting on the spare bush is the water level overflow so I am not relying on the sight glass. Will be removing the safety valve to fill quickly with hot water from a Thermos, save on gas.