You may have better luck if you consider your sushi as a "hand roll." Instant rice isn't sticky enough and the wrong texture for it to make good sushi. Also, you may like it better if you season the rice with rice vinegar and some sugar. Mix everything together except the nori, rehydrate, and fill the rolled nori like an ice cream cone.
If you have time, cook some regular rice (short grain works best), season to taste with rice vinegar and sugar, chill so you can separate it, then dehydrate it. You might also dehydrate the fish you wish to use. I have had luck dehydrating surimi (mock crab), and canned salmon and canned tuna. These you may also wish to season, perhaps with a ginger-sesame salad dressing. Consider dehydrating cucumber and carrot strips, as well. Once rehydrated, assemble as sushi rolls or "hand rolls." If you can find wasabi paste in a tube or powder, either can add a kick. In fact, try the tiniest amount first, to see how much you really want to use... Hubby is a sushi buff and enjoyed these on a trip to Yellowstone.