Directly against the wingtube. Also I built in the receiver on exactly the place you see on the photo's in the manual, so behind the canopy. Lastly both my receiver battery (Kokam 1000mAh 2s 25C) and voltage regulator are also placed here.

I should mention that I use light servo's, I have Futaba 9650 (29 gram) all around, exept for rudder. I built the servo's into the horizontal stabilizor. If you use heavier servo's for elevator, you might not need to place everything so far back.

Also CG is a peronal manner, I have found that I like to fly with a reasonably aft CG, so I don't have it at the recommended 18cm. On really windy days I'll bring my battery like 1 cm more forward to compensete and get better tracking.