Not sure the local welders could help – they’ll tackle most things when asked but tend to burn holes in very thin stuff. I guess brazing might be an option, but I’ll see how long the GunGum lasts first.
gun gum is rubbish, as are the metal collar things. if you look for a local stainless steel exhaust specialist they will make you a section, possibly using your old cat, replace the lot in stainless i say
Personally I would do the job properly, but if cost really is the issue then the ‘Gun gum’ isn’t the main problem its the movement that will make it come off, maybe look at bracing between the good bits of metal so there is no stress & get an exhaust bandage with the wire?