@sabbatical psyllium husks was my current top candidate for insoluble fiber at least. I've used flax seeds in this recipie int he past for both fat, fiber, and protein. It works well but the hemp hearts were a bit better in striking a balance. IIRC the hemp hearts had lower carbs or higher protein or something. I need to check again but i do remember flaxseeds were originally in the recipe but i removed them to get a better macro balance.