Hi Charles,
1/32" x 1/4" or 20g brass bands will be fine on a 3" diameter boiler, I also use 1/32" or 1/16" cork sheet as used for model railway underlay makes good insulation. the standard approach and size for 3 1/2"gauge locomotives . Mantua models sells 1/16 mahogany sheet, ideal for cutting into 1/4" wide strips 
Regards Ray
Edited By Ray Wood 2 on 11/09/2018 18:19:25