Ok, its not actually as good as a Transformer but I finally figured out how to make the carapace open and close without using a bajillion magnets. And it does transform into a Sporocyst. However, I forgot to take a picture so you'll just have to trust me when I say that I didn't glue its bottom on.
I started without building the kit. When I attempted this last time, it was just too fiddly and difficult to cut holes in the body after it was glued together. This time, I cut 'half' a hole into one of the 5 upper body pieces and used it as a template for all the others. The hole is actually quite a bit higher than the joint that the arm should fit into.
I glued the 'arms' into the carapaces for stability and made sure that they would slide through the hole at the correct angle
Then the kit was put together. I drilled into the bottom section of the arm and embedded a 5 x 1 mm rare earth magnet into it. The opposing magnet was embedded into the body. This allows the carapace pieces to be placed in the 'open' configuration.
However, I also needed to make the carapaces fix in place when they are 'closed'. So I made a cylinder out of old sprues and glued it inside the body. I added more magnets on more bits of sprue to get the carapace arms to click into place.
And it's done! With a grand total of fifteen magnets. I have now run out of magnets.