assuming you don't engineer....
you are correct that a "metal" flat washer and a "metal" lock washer are electrically conductive, all be there are nylon alternates they are not needed.
so assuming a "metal" flat washer (with rounded edge, nothing to cut surface) and a "metal" lock washer...
Fortunate enough, never had to deal assembled boards, always was on the surface mount side at the component level. When I was younger I spent many a fun days with a micrometer and calipers, opening boxes from china and ensuring the board mount pcmcia slots were to a networking companies specs.
Aka did you spec the aluminum for the board? if not, great its softer than other stuff you have worked with....again what is your point?
For your form factor board....strip users can use thermal tape :bigjoint: