Heat all syrup ingredients in a pot over medium heat until the sugar dissolves and the butter melts.
Put the cereal in a large (wide and shallow is better than deep and skinny if you can find it) bowl. Recruit an extra set of hands, or prepare to pick up and put down different things a bunch of times. While continuously tossing with a heat-proof rubber spatula, drizzle the syrup all over the cereal, pausing occasionally to sprinkle on some furikake. Even coverage is the key here.
Spread the mix on a foil or parchment lined baking sheet. Bake at 250F for 1 hour, then cool before breaking into bite-sized pieces. It will crisp as it cools. Store in an airtight container at room temperature.