You'll need two baby cucumbers (often sold in wrapped trays of four or five), a piece of seedless watermelon heart (dense and without soft seed-pitting), a sharp knife and a melon baller. A scattering of roasted sesame seeds aren't necessary, but are a cute touch. If you don't care for the taste, you can brush them off before eating.
First, cut off the very tip of the cucumber to make a flat spot. Cut off the ends about two inches up; set the middle of the cucumber aside for use in salads.
Take a small melon-baller (usually it's the end with the spikes) and center it on the top of the pretend "watermelon half". Press it in and turn slightly to cut a good circle, then gently pull one scoop out of the inside of the cucumber.
If everything goes well, you'll wind up with a scoop that's a half-round on the bottom, flat on the top.
You'll want to get an identical scoop out of the watermelon heart, so that it'll make a piece that fits in like a puzzle. Scoop the watermelon the same way.
Fit the watermelon scoop into the cucumber hollow. If it's not flush, you can carefully trim the excess with a paring knife.
Wet your finger on the watermelon piece and use it to pick up the seeds, scattering them one at a time on the surface of the watermelon. And there you go!
If you don't feel these have enough flavor, try adding a drop of lime juice to each one. Also note that these, like other sliced fruits and veggies, are what I call "high-humidity" bento ingredients...if you are putting dry crackers or similar items in the same box, be sure to protect them with a lid or layer of wrap to avoid sogginess.