Cut each squid hood lengthways from opening to tip. Lay flat, inside up, and score diagonally. Cut into 4cm squares. Pat dry with paper towel.
To make the sauce, combine the sugar, grapefruit juice, water and chilli sauce in a saucepan over low heat. Cook, stirring, for 2 minutes or until sugar dissolves. Increase heat to medium and bring to the boil. Boil, uncovered, for 3 minutes or until sauce thickens. Remove from heat. Add fish sauce. Set aside for 10 minutes to cool. Add cucumber. Transfer to a bowl.
Meanwhile, add enough oil to a wok to reach a depth of 10cm. Heat over medium heat to 180°C (when oil is ready, a cube of bread will turn golden brown in 15 seconds).
Combine the flours, Chinese five spice, salt, pepper, ginger and chilli powder in a bowl. Add the squid and toss to coat.
Remove half the squid from the flour mixture and shake off excess. Deep-fry for 1-2 minutes or until squid curls and is golden brown. Transfer to a plate lined with paper towel. Repeat with remaining squid, reheating oil between batches.
Arrange squid on a serving platter and serve with dipping sauce.