If the base of the roller bearing assembly is flat there can be only rotational isolation in the twist direction. 🕺🏻 Plus isolation in the horizontal x-z plane. You lose the isolation in the roll and rock directions. It is the rotational seismic forces that try to rotate the building and everything inside. If the base is concave the component is isolated in those two rotational directions. The smoother and harder the surfaces of the bass and base the easier the component will move when acted on by any external forces. Which means the whole roller bearing set up can easily get “stuck” when the balls roll to the edge of the base if the set up is not perfectly level and balanced. That condition limits the iso device’s isolation, since it cannot move further in that direction. Isolation effectiveness in a given direction is proportional to how easily the component can move in that direction.
Another advantage of a concave surface for the base is that when the component rotates slightly due to rotational forces, there is some degree of vertical isolation since the component moves up and down vertically 🔝 as it rocks and rolls, no? Up and down as it rotates around its centerline axis. Recall wave passing under boat analogy.
You can buy miniature 2” shallow ceramic bowls on line that might work very well and a glass marble of the appropriate size, one bowl on top and one on the bottom. Total cost for set of three roller bearing assemblies $30.