Surely, in the UK, one of the many exhaust parts suppliers do provide a compatible rear box for your car.
I have not needed to buy a new rear box for any VW Group for over 26 years - well except for one for my older daughter's early 2002
Ibiza 1.4 8V, which I had to buy from the local SEAT
dealership, that would be back in 2008, and the price seemed to be "ok".
Maybe look out for a stainless steel version and buy that if the price is okay, £300 sort of frightens me for a rear box.
Finally, are you not sure that it is the just the outer skin that is rusting/cracking, if so, Halfords for example, sell a long stainless steel band that can be tightened around the rear box to keep to within what the MOT tester expects - as long as you catch it before it starts to leak - I did that to my wife's previous 2002 VW Polo 1.4 16V and that worked for many years.