Thanks for your advice Jonathan.

I know what you are saying that aluminium will decrease the moment of inertia as will reducing the radius. I've seen lots of designs that all go the other way and house the ballnut inside the bearing housing using larger bearings still, I'd agree that this is overkill.

This is the design I liked the look of:

castanha 25mm passo 10 da OBR com sistema giratório [rotating nut] - YouTube

What do you think?