Andy.....not trying to teach a principal, however if you drill an on size hole [same as the brass vertical rod] into an aluminium block, this provides a square level surface for placing the washer prior to soldering....yes on the top surface of the washer
The soft solder does not stick or adhere the the aluminium surface....you can also transcribe vertical angles to suit the progressive up-rise fore & aft of the hull surface, together with a MS steel wire placed for athwart rise of hull
Derek