Quick and Cheap and relatively sturdy without using metal (which might not be permitted by the con) or doing a lot of moulding: You could cut "fun foam" sheets into those shapes. For the bits that look raised, glue (I use hot glue gun glue, usually) on a second layer. If you want to paint it a different color, put a coat of gesso on it as primer.
You might be able to use some sort of sheet plastic, but if you want to use glue, make sure it's a type of plastic that can be glued and make sure you use a glue that is compatible with that type of plastic (glue containers will often say what that can't glue).
You could probably also make them using fabric, really stiff interfacing, and then add the designs using paint, fabric markers, embroidery, or quilting. This has the potential to be really ice looking and fancy,, as long as you don't mind having to clip and turn lots of annoying curves and points in those feathers.
If you finish in time, will you be wearing this at Katsucon?
- Ashura (RG Veda); Avatar Kyoshi (Avatar the Last Airbender)
- Ashura or Kohane; Avatar Kyoshi; Minamoto no Hikaru (Otogi Zoshi)(?)
- Kohane Tsuyuri - cherry blossom kimono (Xxxholic)