Any standard extinguisher is going to struggle to put a fire out with fuel from a petrol car. It covers off small fires, dropped cigarette, smouldering luggage etc. For an EV you're unlikely to make anything worse - though I'd avoid a water extinguisher and go for CO2 or dry powder - bear in mind dry powder goes everywhere. A loose fire extinguisher in a crash can be a major hazard, it will need to be well secured. The main HV electrics stay well clear of the cabin and you won't be able to flip the lid to get to the 'engine' bay like an ICE.
Maybe swing by your local firestation for a chat and some professional advice?