Thanks for your comment Brian.
There's a lot of examples of large jigs to drill multiple holes at the same time (like Peter Parfitt's).
Here, after some prototypes, I ended with something different for many reasons.
First, storing a big jig is always a pain. I try to get rid of big jigs.
Second, a big jig takes time to be made, and it's particularly expensive if you print it, if your printer is big enough...
Last, the principle just can not work, because you will need multiple distant reference holes to be accurate,
and a lot of plugs to secure the jig in place. Unsolved problems comes with a multiple hole jig design.
I tried a taller (thicker?) jig, but it did not perform as expected, because the reference is the end your Forstner bit only.
So you can't drill straight. With a shorter jig, you just define the hole location but drill straight problem is the same.
The secondary white (actually light gray) jig takes reference both on the table, the black jig and the drill bit's shaft.
So it is both perpendicular to the surface and concentric with the hole.
As the drill bit shaft is longer than the bit head and distant from the surface, you get a much better precision.
Otherhand, printed parts are very stiff.
I drilled an outfeed table that is slightly bigger than a regular MFT table and it holded the job pretty well.
The jig is still ready for next MFT table tops.