For moving things in a straight, STRAIGHT line, you need linear rails!
Terence at OpenBeam is one creative manufacturer. One of his latest OpenBeam creations is a 3D printer featuring these linear rails, and being resourceful, these rails are noteworthy for several reasons.
Linear rails are designed to move a substantial load in a very straight line, which means they have to be built stiff, and accurately. These rails are clones of similar units made by Thompson/THK/Hiwin with recirculating ball bearings, but are still of high quality. The rails themselves are fabricated from induction-hardened tool steel, that has to be cut with a Wire EDM process (no saw blades will easily cut these!), and are flat to within 0.02mm per meter length, which is an order of magnitude flatter than normal aluminum extrusions.
Where the savings come in is that the surface finish is slightly rougher than the high-end rails, which means a bit noiser operation (but no detriment to accuracy).