Just bear in mind some scrutineers will not pass one mounted under the driver's seat (or indeed anywhere in the driver's footwell) because if the catch is accidentally flicked (eg overalls etc) the extinguisher could roll under the brake peddle.
Having said that afaik many people do run them there, so I assume you need to make sure the release catch is turned inwards and cannot be caught by anything, or vibrate loose.
Also the "when seated" is generally read as seated and belted, so options are very limited in MR2s.
I'd probably consider putting it on the passenger's side of the the "transmition" tunnel as close to the gear shifter as possible; if you can reach it there and if there's something to bolt it to behind all the plastic.