Here's a thought since you've gone this far: Don't rebuild the whole missing portion out of filler. Glue in one or two pieces of "structural" wood to rough out the shape, then do the final shaping by adding filler. Good luck. Post the results for us.
-=tension & release=-
This is just a suggestion....
A serious one though.
Leave the chunk like it is, paint it the way you want (making sure not to get any in the chunk area),
and distress the whole guitar.
Make it look old and used.
I had a 63 SG that, when I bought it, looked like it had been in a swamp for a couple years.
It had a big chunk missing from the edge of the body where it had evidently been dropped.
While I didn't care much for the sound, it did have tons of character 8)
Ken :wink:
"The man who has begun to live more seriously within
begins to live more simply without"
-Ernest Hemingway
"A genuine individual is an outright nuisance in a factory"
-Orson Welles
Talk about an Oh **** moment... I'm with the good CitiZen here. Leave it and label it a "relic". :)
TZ
One thing i'm wondering about is how this happened in the factory in the first place. When I had all of the old patch off, it looked like a forceful break. Maybe when they were sanding it they accidentally ripped a chunk out. That old patch was weird too. How would they have made it hollow?
As for the progress...I got a wood putty that according to the old guy at the hardware store will work the best, and i'm building it up in layers. If this doesn't work, I might try tracking down someone more experienced in woodworking and see if they could make a piece of wood to fit.
Well, I applied the last layer of this wood putty this morning, and it seemed hard enough to sand about an hour ago.
There was also a trench on the back of the guitar with the same filler that I removed and filled also.
The horn came out great.
^The different layers of filler almost look like grain.

There's only this one little spot where the old stuff shows-no big deal.
From a few feet away you can't even tell.
So let's just hope it stays that way when I stain it.
Super job!
"Everybody got to elevate from the norm."
From the pix, you got pretty good results. A polished finish will tell you if it is fair or good or truly an excellent job. What type of putty?
-=tension & release=-
Plastic wood has been around a long time -- pretty sure more than several decades. Are you staining this area, or covering with an opaque paint? If staining, you will need to treat this area carefully -- less wipe-off, some grain faking in the wipe-off that is done, and possibly "locking" layers of the stain between protective coats to build up color as mentioned above.
-=tension & release=-
gnease...I am indeed planning to stain...if you would be kind enough to explain that process in a little more detail, I would really appreciate it.