Boil rice according to package instructions (or use leftover white rice). Do not rinse the rice, you want it to be a sticky as possible.

Trim the narrow ends off each of the cucumbers. Cut each cucumber into 3 or 4 lengths. With a small sharp knife carefully cut out the center seed column. Try to keep as much cucumber flesh as possible.

Julienne the mango, pepper, carrot and green onion. You want the slivers to be fine but stiff enough to be able to push into the cucumber center. You will stuff them into each end of each cucumber piece so you want the slivers to be about the same length as the cucumber pieces or shorter, not longer.

Working from both ends, stuff each cucumber piece with some rice and tamp it down on one side of the hole. Then gently push the julienned mango and vegetables into the hole working from both ends again. Insert as many as you can. The last slivers should be tight to push in, you may not get them all the way but work from both ends until you can't add any more.

There will still be some spaces so take a small bit of rice and tamp it in the end with your finger or a utensil to try to fill the spaces. Trim the ends if there are vegetable ends protruding.

Wrap the cucumber pieces in plastic wrap, tightly and close the ends. Press on the ends firmly to keep pressing the filling into the hole.

Refrigerate at least 3 hours but overnight is even better.

When ready to serve slice into rounds about 1" thick. Serve with dipping sauces, pickled ginger and/or wasabi.