Hi everyone. New to the group. Hello 
I’ve been working on an open source CoreXY printer called S.T.E.V.E for ages and currently working on video documentation for building. Thought i’d share here as i’m sure quite a few of you will be interested even if you don’t build it. http://www.thingiverse.com/thing:2146430
They are hard anodised aluminium, 8mm dia from IGUS. They are super light but not very rigid. Given the amount of torque you can get from steppers, having a super light gantry doesn’t seem to make that much difference. Hope that helps