: How should I fill this void?

: All I can think of is to either whittle a short piece to fill the void (which
: I think will be a real pain) or stuff it with little slivers of wood. Is
: there a better, faster, easier way?

I've stuffed cracks with slivers of wood. You may find a little bit of dookie schmutz does the trick just as well or better. Mask around either side of the gap and force som epoxy & sanding dust into the void with a putty knife then peel off the tape.