With the three Defenders we owned I made the following modification to each mirror.
I drilled two holes in each aluminium arm so I could bolt an alloy plate to the underside of the arms.
These plates extended to present a forward facing "lip" onto which the equivalent of Milenco Aeros mounting blocks could be clamped. I recall I had the lip facing forward so if the mirror was pushed back it could fold, but the blocks of my extension mirrors could accommodate being that way round.
The plates were cut from something like 4 mm alloy, and the bolts were 5 mm stainless steel pan headed.
These were left on each vehicle as they were sold on and I have no photos of the details, but the design had little criticality and something similar could easily be replicated with the Milenco blocks and vehicle to hand.