OK, that's making it trickier. You could always splosh for a hugely expensive car safe. Or if you're going somewhere for a while, leave it with local gun club. Some places will let you keep it with you - I drive a Series 1 Landy, so no locks at all on the car. After a day's shooting we lean the guns up at the corner of the bar, the landlord's got no problem with that. Most of this will come down to common sense, but remember that if you lose it, plod will most likely not be at all understanding regardless of what security measures you took...