Updated 6/18/2019:
I've been giving some thought to another issue. The new piece of sub-roadbed affects the location of a pop-up hatch. Here's the plan. The pen points to the green area (hatch) on the plan.
Here's original hatch outline on the benchwork. Yes, it's triangular in shape.
Here's the area without the new sub-roadbed.
I've decided that I am going to retain the hatch by:
1) cutting it out of the existing table top.
2) putting the new sub-roadbed down on it.
3) tracing the hatch dimensions on the new sub-roadbed piece.
4) cutting the new sub-roadbed piece to accommodate the hatch.
More when I know it.
George