I don't know if i had solved it! But sufficient for me for funny rendering! :)

- Blend the bottom longitudinal arcs for have an arc for close the form
- Nside
- Kill all little parts except the "Ray Manta" and big vertical part
- Copy the longitudinal arcs who are on the same vertical plan
- Blend them
- Extrude both side (the "box" was a try)
- Trim the "Ray Manta"
- Blend the 2 Lateral Arcs
et voilà :)