RE: Grumman XF5F
Well you can easily cut holes through from top to bottom but if it was me I've always wanted to try cutting the holes out lengthways by making the cores in upper and lower halves with vertical spines to hold the shape and so that it leaves spanwise chambers open inside.
The alternate would be to make front and rear shells that glue to a vertical spar. But that way doesn't leave any support for the foam. On a narrow chord like this one you'd probably only need two spines left in and could cut the foam to about 1/4 inch thick. Mind you in this case I'm not sure it would make a lot of weight difference.