I am going to bolt the milled piece on the vacuum table with m8 bolts. The vacuum table has holes for vacuum that are in 120 mm matrix and they are closed when not needed with m8 plastic screws.
Luckily the waste plastic pieces have two 8 mm holes that are 120 mm center to center.
Now I ONLY have to sift my program so that the milling head does no hit the bolts and even uses the existing holes. And bolting the piece makes it possible to mill at least partially through the plastic because I don't have to care about the vacuum.