Place flax seeds in mixing bowl. Add salt, garlic powder, onion powder, and cayenne. Pour in water. Stir. Cover with plastic wrap. Refrigerate overnight.

Preheat oven to 200 degrees F (95 degrees C). Line a rimmed baking sheeting with a silicone mat or parchment.

Transfer soaked flax seeds to prepared baking sheet. Spread out into a thin, flat rectangle, about 1/8 inch thick, using a spatula. Score the rectangle into about 32 small, rectangles.

Bake in preheated oven until flax seeds have darkened and contracted slightly, about 3 hours. Cool in oven with door ajar. Break into individual crackers.