The 25mm alloy from Ullrich aluminium (new Zealand company?) is still the best solution I have found. It’s cheap, rigid, incredibly accurate, and the surface finish is more wear resistant than 316 stainless!
I am in the planning stages of two new MPCNC machines. One very small, parts printed in PETG, and one very large (1200mm*2400mm) parts printed in nylon. Both will be constructed using the alloy tube.
See my previous post for exact details of the Ulrich tube.
Good luck with the build. I look forward to seeing how it comes together.