I seem to have fixed mine with a block of wood.
This tale goes back to yesterday when it wept hydraulic fluid onto my left foot.
Since the master cylinder mount is made of butter and the part to fix it is still in the UK, I had to stop it flexing with something. A block of wood the size of a Jenga piece seems to work.
Bonus is that the pipe junction (bottom left of master cylinder) that I thought was fractured isn’t leaking anymore...winner! But the shop can still have it first thing Monday to make sure.