Some ideas: ask your local framer for some assistance:
Prep a design to give you a rough size.
Ask them to sell you some foam core and mat board cut to your dimensions.
Glue these together with a thin layer of white glue and lots of weights. White side out, leave it weighted overnight.
Your design should use parts of the fabrics as backgrounds, along with the panelling. Your best approach is to ask the framer to mount these materials to your glued board. When you get it back, your background is complete. Your framer should have mounted the fabrics using either Trimount or Fusion 4000 in a drymount press.
Stitch the pieces to the board, using the right colours of thread, then figure out if you’d be happy with the “hard” pieces glued to the backing. If not, use a thin wire. Not sure how to mount the shells, so others here may help.
You’re done. Take it to the framer for framing, or, if you went with a stock size, you may find a deep frame/glass package at a craft store.
Looks like you want to do it yourself, but if you want to do it right, work with your framer. If I had a customer come to me with this request, I’d gladly help.