I have heard that Super Soaker has a backpack for one of it's larger guns. This can be converted to a jet pack by making the launcher out of tubes and Cardboard cones painted red... Also two Styrofoam balls with two cones coming out the bottom attached to the lower corners of each side will make a reasonable pack. It can be painted in much the same way as the armor.
Tips From The Pros: Flynn O'Connor
I made mine from scratch, using cardboard boxes, a 2-liter bottle, and "odds and ends." Painted right, with the right amount of detail work, you can make just about anything look like it should.