Thanks Peter, I'll have a think about it. I'm thinking of adding triangulation instead of the horizontals.

My issue with the big plywood box is figuring out how to flatten the ways for the linear rails. If I use steel I can make the "ladders" separately and machine one on my machine for the ways.

I've read quite a lot that steel/aluminium should be a minimum of 5mm, any less and it resonates too much. Have you any experience of this?