How to open holes in plastic electrical junction box
I would use a sharp knife, a so called utility knife, if I had one handy, but would try my moderately well sharpened pocket knife if that was all I had.

EDIT It may be that all one has to do is to pierce a "ring" with a knife or even a flat blade screwdriver and then pulling propagates the tear as the inside is pulled out.

EDIT2 The fact that the ring feature is convex suggests that the designers envisioned the removal could be with a slicing action with a blade parallel to the plane of the rings. For a right handed person I could imagine holding the box with the left hand with the inside of the left forearm braced against the chest.

The knife would be held in the right hand (cutting edge toward the thumb) with the right elbow braced against the person's ribs. The cut would be made by pulling the knife so that in the event of a slip the right forearm would contact the chest and stop the movement of the knife. After slicing 90 deg the box would be rotated 90 deg to continue the cut . . .

Alternatively, the box could be held down on a flat surface and the knife held with the cutting edge away from the thumb. The removal would be with a slicing action from above then the box flipped over for slicing the remainder.