Rail (in black) is 1.75" OD 0.25" wall DOM mild steel with a bushing finish (available for about $6.00-8.00 a linear foot).
Support (in blue) is 1" high by 3/4" wide HDPE plastic with a 1.75" diameter receiver (?? is that the right word?) milled with a router into the top. What I mean is that the profile of the tube is routed into the top of the support creating a cradle for the tube.
Large bots pass through the frame into tapped HDPE to secure the HDPE to the frame.
Small bolts pass through the HDPE (and are countersunk into the HDPE) into the tapped DOM tube to secure the tube to the support.
Spalm I think you are right, steel bearings on a aluminum tube would gall the aluminum.