I use a hole saw to cut the wheel well then I make a verticle grained balsa box in the core. The balsa box butts up against the spar. After the core is skinned the wheel well and retract box is opened up. A ply plate is fabricated to mount the retract and installed into the box. The exterior is finished off with some balsa. When joining the wing, a vertical grained balsa/CF joiner is used and butts up to the rear edge of the spars. Pictures are worth 1000 words.