Not sure but I would say steal a small piece of foam from a soda cooler or similar, squeeze it in place, glue in place with foam-safe CA, and sand smooth.
That's what I just did. Took some foam that I found in the basement, cut the piece square on the plane, cut the donor foam to size and then 30-minute epoxied it in. I'll cut and sand down when dry. Now I have to piece the rest of the plane back together! I'm also going to order a new tail section to use as a template to fabricate it out of balsa and see how that works.