I believe it has not been done before because a handed pair of tools to make the base will cost thousands ££££
Even a pair of prototype tools (very limited numbers) with a consequent reduction in surface finish quality will still cost £1200-£1500 per tool.
BMW did not tool up for rhd production.....numbers made versus investment cost ?
I guess that leaves machining from a solid billet of HE30-HE40 Aluminium.
Thats some serious machining !
The major concern is splitting & reassembling the unit.
The mirrors are an assembled unit to the customer.
I think separating the two will possibly damage the lens housing.
The base & its two leg dowels are pressed to the main mirror lens housing.
The pins look like they have been cooled before assembly into the housing.
It might be handy to have some liquid Nitrogen around