There happens to be a "microballoons" made to add strength and lighten up the resulting mix. If I remember correctly, it has "phenolic" in the name. I'd run down to the shop and check, but done that a couple of times already. I think Sig sells it.

So the best microballoons mixture for medium repair would be that stuff if strength matters. But someone mentioned scarfing in balsa, and that'd be the best to do and then use the mix to glue it. It'd glue and fill both, allowing the filler wood fit to be less than perfect. Save time that way.