My friend John Scherer is an active member of local hackerspace TCMaker and he is building a super sturdy small CNC router on a shoestring budget. They don’t have a milling machine there, so I helped him out.
The rails themselves are aluminum extrusions with what looks like 4 steel rods embedded where the ball bearings contact the rail. Very clever.
The plan was to mount them to the 80/20 style extrusions, but as can be seen from the photo above, the channel neede to be widened a bit for the extrusion to sit flat. Holes needed to be drilled through the 80/20 for mounting screws, and steel backing plates made, since the 80/20 is thin in the center.