I can tell you how I'm doing my servo set up on a plane this size... I'm going sheet the bottom center section of the wing with 1/16" balsa. Top will be sheeted as well with 1/32 ply hatches. On the inside of the wing I'm going to run 2 1/8" x 1/4" spruce rails between the ribs. Next I'll cut an opening for the servo and mount it from the inside of the wing...

I think it'll work pretty slick with out adding much weight. I'll take some pictures as soon as I get it done.