I'm sure its all been done before but here are some " block plane " cutters I made to cut in a spare on the foam wings I cut last weekend..

1st attempt was a square spare so I made a cutter of the appropriate U shape.
This made a very nice cut into the pink wing.

2nd attempt was to create a cutter to embed a rod into the wing and leave only one cutline showing.
I think this is what I will use on the final wing..

Screws on the end of the " plane " apply pressure to the wire so it doesnt move. Depth of cut can be adjusted here.

steve