When I did my fins in plaster, one of my molds broke in half. My response was at the time. *uck im not doing this again, where's the glue. Making molds is so time consuming and pouring them with Silicone is even more consuming. As fins are an odd shape they can break its only natural with plaster, however my experience has told me to make the molds as thick as you can using what you have.

Now with my fuke I did I ended up using fiberglass. it worked extremely well however the negative to these are, if you make them thin they will only last for the duration of the pouring process as removing the poured object takes its toll on the cavity itself. also the CAVITY needs to be place on a level surface otherwise it will warp. Fiberglass resins are not cheap especially here in Australia but it done the job. if Made thicker they will last, so being fins are an odd shape fiberglassing could be a better option.