In a small bowl, mix 1 tbsp rice vinegar, ¼ tsp sugar and ½ tsp salt until dissolved.
Mix the vinegar mixture with 320 g cooked Japanese short-grain rice while it's still warm. Gently fan to cool and cover with a clean damp tea towel until it's time to use.
Next, place 1 Japanese or Persian cucumber(s) on a chopping board and sprinkle with ½ tbsp salt. Roll back and forth until fully covered.
Wash the cucumber with cold water and then pat dry with kitchen paper.
Cut off both ends, and peel the skin off of the top and bottom. (The skin at the ends is often thicker and can have a slightly bitter taste.)
Cut the cucumbers in half lengthways, then again into quarters. If your cucumber is large, you'll need to cut it into sixths or eighths. Make sure every piece has skin on it.
Run a knife along the area just below the seeds to remove them.
Fold 2 sheets sushi nori seaweed in half and pinch along the crease so that you can neatly tear them in half. (You can use scissors if you prefer.)
Place the rolling mat infront of you so that the bamboo is laying horizontally. Set the nori down with the rough side facing up, line up the bottom edge with the bottom edge of the sushi mat.
Wet your hands with cold water. Keep a bowl of cold water nearby to use when necessary. (This helps stop rice sticking to your hands.) Take a handful of sushi rice (approx 80g) and shape it into a rough cylinder.
Leaving a finger-width border along the top of the nori, place the rice on one side and gently spread it to the other side. (You can use a rice paddle to spread it if you prefer.)
Use your hand to make a wall and push the rice right up to the edges of the nori. Make sure to leave the border at the top clear.
Make a ridge with the top edge of the rice (next to the border).
Add a small blob of wasabi and spread it down the middle, then place a strip of cucumber on top. Gently press it into the rice to stop it from shifting when rolled.
Lift the edge of the bamboo mat closest to you (the bottom edge) and carefully roll it over so that the two edges of the rice meet. (Make sure it's tight!) Lift the bamboo mat slightly and roll over the flap of nori to close the sushi. You don't need to wet it down, the moisture from the rice will help the nori stick together.
Once you have your completed sushi roll, use the bamboo mat to press the edges and shape it into a cube. (Kappa maki is typically a cube rather than perfectly round.) Repeat until you have four rolls.
Wet a sharp knife with a damp cloth. Cut each sushi roll half, then cut each half into thirds. Cut using a short, sharp sawing motion and wipe your knife between each cut.
Enjoy your homemade kappa maki with soy sauce for dipping!