Immobilizer (suggestion needed on "remote").

I'm completely new to Arduino but it seems I should be able to accomplish what I need with it. I'm a bit rusty on the HW side of things but the SW should not be an issue for me.

I want to build an immobilizer for a car that I'm also building (I.e. I will have full control over the wiring and parts used).
Me key, no pun intended, requirement is that I should be able to start the car with a "fob" in my pocket. I should also not have to press a key on the "fob". It should be completely passive. The reason being that I will be in GRP bucket seats and if I forget to press the key it will be quite a bother to get up to get access to my pockets. The car also has no doors so I would not have that reason to use the "fob".

The plan is to have an ignition switch that will power up the Arduino and then it in turn will power up the rest of the electronics in the car after a handshake with the "fob". I can then press the starter button and of I go...

My problem is that I don't know what to use for a "fob". RFID does not seem to be an option as it will not be close enough to an antenna. I don't want to use BT together with my phone although it could be a backup solution if nothing else works.

I had an idea that I could reprogram this FOB: but I'm not sure I could get the power consumption low enough without being able to start the car only every half hour... It would otherwise be a nice option as I will already have a transceiver on the Arduino for another function that I'm adding.

Looking for suggestion on something that will be small/rugged enough to have in my pocket and with a battery life of at least 8 months (it will be a summer only car).

Not sure this was the best part of the forum to post this so if an admin feels it should go elsewhere, please feel free to move it.

If you're concerned about security, are you also going to have a steering lock? Are you going to provide strong security and anti-replay attacks on whatever wireless interface you adopt?

Making a good security system is hard. Making a bad one is pointless.

Security will be a none issue for this other than being able to show that some sort of "key" is needed to use the car.

Would a battery isolator switch with a removable handle be considered as a 'key'? Based on your description I'm guessing you'll have one.

Yes, there will be a FIA switch. However this is in addition to that.