JR, a dremel works good for channels in Foam with the "Tile grout removing" bit. I tend to install wood in foam with poly glue, and anything that gut cut too much gets filled in during many different steps of the process. "Hog it out" is a phrase I am accustomed too.

If the tile bit is too short, a strategically inserted 1/8" drill bit works well also! I have a dremel router attachment that I can set depth of cut.

Looks good,

Sean