Buy a tube that fits the wing. Enlarging the holes in the wing cores would be a nightmare to get them straight and lined up perfectly. You most likely wouldn't get it perfect and then you'd be mixing up a slurry of epoxy and microballoons to fill in the gaps. Extra weight and not good building practices IMO.

Don't do it. Get a tube that fits the cores and phenolic tubes you have on hand.

The $8 extra cost for the right size tube is worth every penny you'll save in frustration trying to hog out those foam cores.