Estes Black Brant XII, Kit # 9723 Build, Part 13, Tunnel Fit To Tube

You should sand a curve into the bottom face of the tunnel for a tighter fit against the body upper tube. 

Start by drawing a pencil line down the center from end to end.

TIP: Press a "starter" curve down the center pencil line using a rounded dowel end. This slight indentation will help you keep the sanded underside curve straight.

TIP: One trick to getting a good fit on a tunnel is to wrap sandpaper around a smaller tube than the one it will end up on. Example - If the tunnel will be glued to a BT-60 tube, sand the bottom curve with sandpaper wrapped around a BT-55 tube. 

Here's the before and after fit of the bottom of the tunnel.

The outside of the tunnel was rounded off using 220 grit on a  sanding block for the rough shaping. 

400 grit rolled under my fingers finished off the rounding.