Aerodynamic bike helmet!

Step 5 Make two more cross-sections.

Dust off your imagination and use it to envision a cross-section that's one foam sheet away from the original one. I chose to cut this piece to match one of the vents in the back of my helmet (see photo) so that I would have an easier time with assembly. After you've decided on the shape, cut out a second identical version to go on the outside of the other sheet (e.g., in the picture we can see the cutout that goes on the right side of the helmet; a copy of this cutout will be attached to the opposite foam face).