Here’s the way to say “diaphragm”:
横隔膜 (ōkakumaku: diaphragm)
side + partition + membraneHard word, but it’s fun to say!
As for “solar plexus,” I can offer one 胸-related way:
胸先 (munasaki: solar plexus; breast; chest) chest + end
Here are two interrelated options that don’t include 胸:
水落 (mizu-ochi, mizo-ochi: pit of the stomach; solar plexus)
water + to drop鳩尾 (mizu-ochi, mizo-ochi: pit of the stomach; solar plexus)
pigeon + tailWith the second word, we’ve got ateji. With both, we have a whole lot of strange things happening! Wish I understood!