Tried a few options but thought I’d share the best one I’ve found.
It uses the constant 12V power available via the OBDII port of the car. Perfectly designed to fit discretely and not get in the way. Also has inbuilt fuse for added protection.
Can’t find them here in Australia but they are the Kable Hi5 Adapter. For the Model S, if you want the USB port facing towards the front of the car get the right angle version (which is what I got - USB stick is hidden nicely). If you want the USB facing towards the drivers seat get the left angle one.
Link to the right angle via a supplier is here: Products
Hope this helps someone else. Easier than wiring in.
Shane.