Not a good photo.. however you get the idea. The extensions off the set runners are 10mm x 10mm solid aluminium bar (connected via the forward bolts holding the seat)... and fitting the extinguisher here also means no holes in body work etc.
knowing the bonnet is a b***h to open i would try to have a extinguisher mounted under the bonnet with a swith or pull mechanism inside the car. you could even prolong the tube. that way the extinguisher can stay under the seat rens