BTW, if you think the mass of the servos is enought that they might not need to be moved back to where they are usually placed by almost every mfg, then the CG can probably be moved enough by shifting the battery just into the fuselage aft the wing bolts. Or as far as you can reach to glue in a balsa box for the battery if it's already just aft the wing bolts.

On the other hand, why not just move one servo back under the stab and see how that works.

And if you KNOW moving both servos ain't gonna hack it, then go ahead and move 'em one at a time to find out how much more weight shift is needed.

Do measure the CG locations however. It's often amazing how little we learn from some things that could teach us so much.