I use the baking soda trick quite often.

Use dry baking soda, fill the hollow/dent/crack/etc with it. Soak in Thin CA. Stand well back from the fumes genrated. Give it a few seconds to cool down.

I have recently used this technique to:
fill the holes at the exit point of the rudder/elevatorcontrol rods from the fuse.
When building an Engine Cowl from Balsa, lots of joins were difficult to match perfectly .... who cares. Fill the mis-cuts with backing soda. Drop some CA on.

The only real problem is that it cures very very very hard. Almost impossible to sand right.

gus