BS 1501(old grade from the 70s sure it had changed no.s last time I bought some), is wot iv used in the past for surprisingly boilers tho that would be very OTT for a burner, mostly bought because its certed and used in such apps.......wouldnt worry 1/4" M/S would be fine if you went thinner id be tempted to add some stays if its square......
now there was a rod used for boiler plate but the no. escapes me 7024s maybe anyone?