Jonathon,
thought about that one last night but I would need to get it accurate. Flat bar bent to a right angle with a 12mm tapped hole, then G-clamp the long bit to the side of the extrusion. I have already machined the linear bearing guide blocks which will be bolted to these sections.
Ive had a look on the suppliers website and they say they have self threading core screws but dont give any lengths on them and they are still on holiday. Also they would need to be cap screws. I think a spiral flute tap would be easier once I get one.
I just hate having to wait on things.

Ian