It is possible to do it later on but it is important to say have 4 reference holes in the underside of the playfield so when you come back at a later stage you machine these 4 reference holes mirrored in say a bolted down piece of MDF. Then place 4 dowels in these reference holes and then slide and lock in the pf over these dowel into it's reference holes and bolt this down.

With the pf zero'd in do a air cut above the playfield as you would to stuff it up this late in the game and check your location for the 3D v cut and slowly drop the z down until you are done.

I have done this alot when I needed to machine both sides of things but before you go to do your first stage this weekend add 4 x reference dowel holes preferably in each corner.

Hot that helps

going well