Our flue is a weird shape and the cover fell apart so I wanted to rebuild. I was thinking about going with 1/8" angle iron verticals and brazing them to the top. I'm pretty sure it would outlast me. I haven't done it because it just seemed like it wouldn't be any fun. 3/16" stainless would be really annoying to braze with my oxy/acetylene setup, and if you don't have nickel silver, it would be expensive. I don't know how much heat your torch provides, so it's difficult to say.

My wife doesn't want me to buy a TIG welder because I might make something, so I was going to join the local maker space and use theirs.