Cook the rice. Not including instructions here, but if you need them, Yumiko over at RecipeTin Japan has the perfect recipe.
Take however much rice you need when it's done and transfer to a bowl -- let it cool for a bit, about five minutes.
Moisten your hands with water and take a handful of rice, forming it into a ball. Make a well in the center of it and spoon some of the plum inside, or tuck in a whole one.
Shape rice around the plum, forming a triangle or pyramid with rounded corners. Repeat however many balls you want to make.
Grill rice triangles/pyramids in dry skillet or grill, turning them carefully to brown on all sides, about 10 minutes total. Brush with coconut aminos and grill a bit more to crisp the rice until golden brown.