rough sketch of my recent ideas:
Which means that i will mostly keep the shape in the lower part of the back, but even out the curvature of the ribs. In the top part i decided to leave the back light free (for legality..). The silhouette was rising toward the back which i do not think is advisable - an early mistake by building the box, and a good opportunity to correct it.
I also plan to use planks from light wood (Paulownia) as the cover and run these parallel to the back plate of the velomobile. Which should be both optically appealing as well as easily to be fitted using a small plane.
Different issue - the wheel box within the back. The dirt pattern developing in the box shows large regions where i do not need any protection and can cut off a large amount of material to reduce weight. But how might that affect the drag of the wheel in the box?
