Just made one for my 1968 Plus 4 out of stainless. Total material cost was 4 quid for a 12" off cut of stainless round.
Took around 2 hours including screw cutting the stem, drilling, tapping and polishing.
Popped the 50 year old alloy disc in the lathe and give it a light facing. Looks as good as new
Arwyn
