Regarding the sacrificial board, you are free to do whatever you desire. The current design has, as you point out, two “tops”. The “first” top is actually part of the torsion box, glued-and-screwed into place – providing mechanical integrity and rigidity to the base assembly. The “second” top is just screwed onto the torsion box. Shit happens, tools to go deep, the base gets damaged. My sacrificial board is getting kind of messed-up with cutting oils. Also, I suggest cutting “t” slots in the sacrificial board to enable clamping your work piece. I wouldn’t suggest putting those tracks in the torsion box – just seems to me that this would weaken the structure considerably.