What's the best way to fill large holes in wood?

Buy a can of foam insulation. Spray some in there...not too much...about 3/4's of the way...it will expand the rest of the way and maybe more. Once it sets for a day and hardens....you just come back with a long knife or even a hacksaw blade and trim off the over flow. It will have bubbles that will need to be filled after that...so use some regular latex painters caulk with a wide blade putty knife.....slap some on...smooth it out with putty knife. Once it dries you can then paint over it.