You can decorate a multitude of cookie shapes into a watermelon slice.
Beyond a watermelon slice cookie cutter,
you can make your own with a circle/oval cutter and a straight edge cutter.
You'll need 10 second glaze
in 4 colors: green, reddish-pink, black, and white.
If you'd like your watermelon seeds to be more 3-Dimensional, make transfers.
On parchment paper, pipe black dots and pull a toothpick through the top to form a seed shape.
Allow them to dry overnight.
Outline and flood the rind of the watermelon.
After it has dried for 30+ minutes, add a white outline inside the rind.
Give them 30+ minutes to dry and then outline and flood the inside with the reddish-pink glaze.
While the glaze is still wet, carefully set the "seeds" in place.
OR......if you didn't make transfers, just use the wet-on-wet technique.
While the reddish-pink glaze is still wet,
add black dots and pull a toothpick through the tops to form "seeds".
Once the green glaze has dried overnight, paint random stripes on the rind
with your choice mixture of green, white, electric green, and lemon yellow Americolor gels.