Estes Black Brant XII, Kit # 9723 Build, Part 14, Detail Hatch

Use the extended lines on the full size drawing and transfer them onto the balsa detail pieces.

The same as the long tunnels, 

Draw a pencil line down the center.

Using a rounded end dowel,

Press a "starter" trench into the balsa. The slight rut will help keep the piece straight on the body tube. 

Wrap some 220 grit around the tube and sand in a straight line up and down.

On the right side picture, you can see the sanded curve in the underside. There is still some of the flat bottom left. Sand until the curve is complete from side to side.

Here's a good fit of the hatch piece against the body tube.