Rolled sushi or maki sushi is generally pretty good at staying together the nori goes soggy quickly which helps the roll stay together. Some types need a bit more help keeping it together.
One of my Current favorites is using Tamago to make Nigiri sushi but it does need help to stay on, usually a strip of nori works quite well but for extra traction I was fiddling around and came up with sushi cement. Its very simple and serves the dual purpose of holding the topping on the nigiri and being tasty too. I'm guessing for all you carnivores out there it would mix well with tuna.
Sushi cement.
2 spring onions, roughly chopped
2 tbsp QP mayonnaise
1 tbsp toasted sesame seeds.
place the spring onion in a food chopper or processor and pulse. If your seeds are not already toasted simply swirl them in a hot frying pan until they begin to pop and colour. Mix everything together and use how you see fit. Feel free to change ratios depending on what you want it for.

