Is the extinguisher pictured have a plastic bracket?, looks like it, but uncle col sees as well as Mr Magoo.
Sometimes its easier to get the extinguisher refilled, rather than buy a new one and go through the process of refitting it.
I went through this process a few years ago for the 156.
Went to Bunnings, Mitre 10, Repco, Supercheap, Autbahn, etc and ended up finding one that was ADR Approved, one month old, steel bracket and would fit between the runners on the seat and the seat could still slide back and forth.
Don't buy the first one you see, you would think that they would be all a standard size, but strangely no, spend a bit of time finding one that suits your car.
And hope you never have to use it, Colin.