I used some Alu stock bar 120mm x 40mm x 15mm.
I drilled (and tapped) the mirror mounting holes first and then drew around the mirror foot.
They were then roughly cut to shape with a hacksaw and finally filed to size.
The first hole (nearest the F) is 7mm dia, this takes an M6 Hex Headed Socket Screw from the front mirror mount into the front mirror mount on the door.
The larger (circular) hole is also 7mm dia but counterbored to accept one of the original mirror mounting screws (countersunk cross point), this screw secures the adaptor into the rear mirror mount on the door.
The last hole is tapped to M6 and accepts an M6 Hex Headed Socket Screw from the rear mirror mount into the adaptor.
Naturally the large hole in the centre is for the mirror cabling.
You need for this task:
2 x Alu stock
M5 Drill
M6 Taper and Bottom Taps
M7 Drill
M12 Drill (for the counterbore)
A hacksaw with large tooth blade (8 or 9 TPI)
A selection of coarse files (you can use fine but they clog with the Alu).
Several gallons of Tea, some good music and a few hours (for filing).

When the temp. climbs above zero I'll fit them and post more pictures.
Then I'll fit Smart washer jets to the wiper arms.