Robert, just a thought and certainly not a criticism, but I'm guessing that the original brackets for the Sinclair mirrors are designed to allow the mirror to fall off on contact without causing any damage to the bodyshell.
I'm assuming that if you clamp the mirror to the bodyshell too strongly then in the event of a prang either the mirror and/or bodyshell will potentially suffer greater damage than if it were allowed to "pop off".
Polished aluminium will look really cool though