Place the makisu on a chopping board, cover a makisu with plastic wrap and place a sheet of nori on top.
Spread rice over the nori sheet, Sprinkle sesame seeds over the sushi rice.
Turn the sushi layer over.
Lay the cucumber and tuna slices, smear a line of Garlic chill source and mayonnaise along the length of the nori sheet.
Carefully begin rolling the makisu from the edge nearest you, using your fingers to keep the fillings in place. Press down tightly and evenly, but not so tightly that the rice.
When the makisu has almost completely enveloped the sushi, pull out the end of the mat so that it is not rolled up in the sushi. Holding the makisu around the sushi, pull the other end of the mat gently.
Cut the roll in half using a sharp knife into 6-8 pieces.