Chris,

One would think that lining the servo cut-out with balsa was a standard practice? It certainly stiffens the whole area up. Which is what I think you're after.

I would use 1/16" balsa to line the cut-out and then make a small servo tray out of 1/8" ply. If you wanted to you could then use some balsa on top of the ply plate to fill any gaps for a neat fit around your servo and sand flush with the skin.

Probably many ways to skin the cat and this is probably why Mike has not said too much about it in his manual...

Cheers
Jason.