Very nice modification. Replaced 8mm rods with 12mm in my printer. Now table is much more stable.
(Z axis solution (for heavy table) used in X3D, where table is supported only on one side, is bad one anyway; causes small bearings lockups etc).
Notes for others:
- mount holes for bottom part do not match original holes of X3D XL (not a big problem)
- I used 2 LM12LUU (long) instead of 4 LM12UU (short) bearings; longer fit without any problem
- bed support needs to be printed with supports unfortunately