Autosafe's Satellite S-5 safe is designed to fit in your car and prevent opportunistic thieves making away with your expensive kit. It's large enough to hold your PDA/GPS system, sunglasses and iPod.
There are no permanent fixings to attach to your car, so you can move it around with you. To fit into a car, you just loop the security cable around an existing fixing, such as a front seat or chassis-mounted luggage loop, and lock it into place with the S-5's lid. The safe has been verified by Thatcham as being able to withstand a five-minute sustained attack, which will prevent all but the most prepared thief making away with your kit.
It might seem expensive, but it's tough and it's cheaper than paying to replace stolen products.