The base for the linear bearings are cold-rolled steel plates. The one shown here is a 3-foot long plate 1/4” x 6” for the Y axis. A similar plate, shorter and only 4 inched wide is used for the Z axis.
It is bolted to the gantry box with machine screws. 1-inch square spacer bars behind the plate (8020 profile) leaves space for the bearing blocks.
The steel plates were purchased from http://www.onlinemetals.com.