The way i see it if i had laser beams bouncing of mirrors that are moving, i would want not only rigidity but as smooth running as possible. Linear bearings may be great but there is still a lot of movement especially in small component form. In my opinion plastic or bronze bushe bearings would be my choice. I have used home made acetol bearings for my 3d printer build and work very well. Igus bearings would be better than because it allows for thermal expansion. With no real cutting forces going on you can get away with quite a lot...Drill some wood and give it a go, you will see what i mean.