I followed this method back in Dec. 2016, and it lasted till recently. However, the bulb on the end of the stalk has become progressively slack over the years (and probably about 10-12k miles) and I couldn't use the dip lever without having to readjust the mirror.
I tried one of these TRIUMPH STAG,TR4A,TR5,TR6, 2000,2500,2.5PI INTERIOR MIRROR MNT Bkt & ARM https://www.ebay.co.uk/itm/TRIUMPH-STAG ... SwE4hdq2WC
as a replacement, but the arm's too short. So, unless I set the seat forward and lean right beck, I can only see a short distance behind me or the underside of the roof. People who are short in the upper body might be alright, but if your anywhere close to 6ft or above, it's not likely to be a good solution.
I do have a couple of spare Dolomite arms and while looking for a spare Stag mounting I thought I had I came across a TR7 mounting and arm, which has the same spherical hole. So I pulled the arm off that and and the Dolomite stalk fits.
The top side is wrong, as there's a hole in the TR7's headrail for the back of the bulb and the holes are too close together by about 2-3mm. However, with a bit of work with a black glue stick in the hot glue gun and a file to move the holes, it now fits. It's not perfect, the shape does not match the mark on the headlining and you can see the join between the plastic and the hot glue a bit. But it works fine.
However, TR7 ones aren't much more available than Dolomite ones - there's currently just one on eBay in the UK. I just happened to have one at the back of a draw. So it still needs a better option for a replacement.
What would do is a stalk like the Stag one, but about 2" longer.