Make shim for cross-member beam

[griswaldbrooks]

As per #3
@griswaldbrooks and @tamarum will make metal shims for the cross-member beam, because it's shorter than the 80-20 beam above it. When we get the shims hopefully for the next meeting, the cross-member beam can be assembled back into the CNC structure/motion axis, and then the rest of the fasteners can be tightened.

Need to get measurements for the shim.