Not knowing the MkV personally, is there a clamp that can be tighter.
This is rather too obvious ?
Something I have used in such cases is something called 'muffler putty'.
This comes in a little tin, and is a thick gooey substance.
Once smeared onto a pipe or muffler joint and heated a bit, it sets like a rock.
And can 'glue' things together quite solidly.
(A rubber mallet can usually free it off)
Worth a suggestion ?
If it remains problematic, a self tapper drilled into a strategic location ?
Which does nothing for originality, but if its that or lose a muffler...