Heat nori in 350° F oven to soften slightly, about 3-4 minutes. Rub avocado with lemon juice. Working with one sheet of nori at a time, place the shiny side facing down and the long side facing you. Spread about 1/2 cup of the rice in an even layer on each sheet. Leave about a 1/2 inch border on each long side. Arrange some avocado trips horizontally across middle of rice, followed by a layer of cucumber and crab. Dab crab with wasabi. Roll and cut into 6 pieces each.