I have a separate 400 sqft building on my property and I am adding some circuits to it. It has a 125 amp subpanel and power to it is supplied from the main panel by cable which is run underground in conduit, enters through the bottom of the wall into a large, plastic box, and then up inside the wall to the subpanel immediately above it. Below are two pictures of the arrangement:

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I would like to bring several cables through the left hand side of the plastic box and then run them up through the top of the box to the subpanel. The plastic box has no knockouts, and is 14-1/2" wide, so it fills the stud space completely from side-to-side. When I drill through the plastic box and through the stud it is up against, do I have to somehow install a grommet to protect the cable where it goes through the stud and the 1/8" thick plastic which makes up the side of the box? Thanks.