Having looked at your pictures, I had the same trouble with my windscreen
corners though they were not quite as bad as this.
I had to have the windscreen replaced recently and took the opportunity to
cuty out the rot and replace it with metal & mig.
I found that I was able to make a good copy of the metal shapes involved by
taking a cardboard cut out of the other corner (which was not so bad) and
then making up an assembly of little plates that I then welded together.  It
took about 4 hours in total to repair, I was able to grind the area flat and
then paint it with some rust preservative before the new screen was placed
back in the hole.
Suggest you remove the screen before you try any of this.
Your screen will pop out easily (especially with so little metal holding it
in place) if you kick it in the bottom RH or LH corner - but have somebody
ready outside to catch it - mine came out first go with one kick.

Suggest you get a big reel of mig wire & a large bottle of gas before you
start.

Good luck.

Phil, T25, now with an additional inline heater fan (thanks Mr Bulley for
the idea) so it is hot.