Use a piece of sand paper wrapped sharply around the side of a small scrap of 1/4 inch plywood. Cut the plywood into a rectangle about the size of an index card.
100 to 150 grit should be fine. The square sides and edges of the plywood should follow your molded edge nicely, and be thin enough so you can do one side of a channel, and then then slide against the other wall to do the other side of the channel.

Very light pressure is all it takes to remove the foam bits. They should just fall apart.

PGJ