So I used a 90 degree drill to do it. Started small with a #45, then went up in stages when I was happy with the hole.
It’s a strange one as the hole is not in the middle of the longeron, it’s in the middle of the horizontal vertex when measured from underneath, and you can’t see the longeron as it’s buried under a load of other metal.
It took me 4 weeks to build up the courage to drill those holes!