Just to finish off this thread in case someone else has this problem in the future:
SIG responded quickly and suggested I use magic marker on the dihedral blade to mark the high points, shine a flashlight down the wing slot, then sand off the high spots.
I couldn't see anything past 3-4 inches down the wing slot.
So, I made a sanding stick the same width as the dihedral blade and tried that. No joy. So then I made a tool from a thin piece of plywood with a "spade" X-acto blade on the end of it. This I worked down the corners of the slot several times. Eventually, after shaking out some junk and running the sanding stick down a few more times, the dihedral blade finally went to the bottom of the slot. The wing halves are now joined (until crash do they part). Dzl