Which grade aluminium?

[Jonathan]

Flat aluminium bar is nowhere near flat, but if you get standard 6082-T6 flat plate I've found that to be very close. If you're using supported round rails it will be sufficient, but with profile rails greater flatness is required to stop them binding so I'd go for milling pockets in the flat plate or if you've not got a milling machine get the eco-cast.

For example:

Edit: Ecocast is no easier to machine it's exactly the same to work with as 6082T6 it's just precision ground flat.!!

Surely it's milled not ground, not that is really concerns us.