So I done a bit more search, I can get something like this ready made
https://www.jaycar.com.au/150w-car-l...upply/p/MP3472
Or,
I can get one of these and wire it up to a ciggie adapter one end and laptop plug adapter the other
https://www.ebay.com.au/itm/150W-DC-...QAAOSwPgxVMMMj
I also need a 7amp fuse.
Anything I missed?
Bo