Scoop out 1 tablespoon of watermelon. It doesn't have to be exact.
Slice the watermelon into 5 mm matchsticks.
Separate into two batches and coat one half in yuzu pepper paste, the other in miso.
Let the watermelon sit for a while to absorb the seasonings.
Cut the chikuwa in half, then diagonally cut each half again.
Stuff chikuwa sections with watermelon from Step 4. With 2 to 3 pieces each, the watermelon should fit nice and snug without falling out. In the above photo, the left group is seasoned with yuzu pepper paste, and the right with miso.
Of course, you can try it without any seasoning.
Here are three varieties: (clockwise from the front) plain watermelon, seasoned in yuzu pepper paste, and miso.