Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Soak shiitake mushrooms in hot water until soft, 5 to 10 minutes.

Meanwhile, lightly grease a 9x13-inch baking pan or glass baking dish. Spread rice in the prepared pan; sprinkle furikake evenly over the top.

Drain mushrooms and squeeze out excess water. Mix mushrooms, imitation crabmeat, mayonnaise, sour cream, tobiko, and kamaboko in a large bowl. Spread over the furikake.

Broil in the preheated oven until the top is lightly browned, about 15 minutes. Slice into 24 portions and serve in the pan. Spoon a generous mound onto a sheet of seaweed, wrap loosely, and consume immediately.