uh.... maybe seashell conch/cone?
also some of them have some spike to it that can help against bite or contact, maybe put some anemone to it too if you want to add extra protection.
this two example are made of metal so dont take it too literally, but there exist conch as big or wide enough to fit a head (cant upload the image, so i just put the link)
and here some example image that show a conch big enough to fit a person head that i can find.
oh yeah also clam (dont take that to literal, although real clam that big is easier to find) you can also use smaller one and link it to become maile coif or scale coif (can be applied to small conch or cone too as willk show in this question using cowrie shell).
otherwise coconut shell or fiber is one of the option, some of them likely drop and drift on the ocean for easy picking or if your triton can move near shallow water the shore likely have some of it.
helmet made of coconut fibre preferable as padding
also pufferfish to add that extra poison as an addition to the helmet outerlayer.