The other half has a corsa and the Exhaust Rear Back outer shell has corroded. Can i use filler?
Body filler?
no if so,it will just all break off, you need something heat resistant,
Gumgum etc,but this burns off eventualy too,
for agood bodge that lasted awhile ive used metal gauze/Mesh this is widely used for bodywork,
put mesh over hole in box and cover it with JB weld,
or as dave says buy new box, 25note's ish, bodge 'in materials cost up too
10 quid ish so, its false economy realy, (fix not bodge is the best aproach)
Edit' just re read thread, (corsa) these are one peice system so isees why your bodge in,
find eggsaust place that welds, cut box off and weld another one on,