You could use a 30mm hole saw, weld a plate with the nut in position then weld the hole up again.Yes, saw that. With the double angle bracket I was given I will be able to move the bolt hole just that little bit further out so that the hole (and therefore the nut) is between the outer closing panel and the outer sill panel that I can access from underneath. Your spot is between the inner and outer sill plates, difficult to access. Will play with it and take some photos.