Ultrasound mid-air haptic technology provides a large space of design possibilities, as one can modulate the ultrasound intensity in a continuous 3D space at a high speed over time. Yet, the need for programming the patterns limits rapid ideation and testing of alternatives. We present Feellustrator, a graphical design tool for quickly creating and editing ultrasound mid-air haptics. With Feellustrator, one can create custom ultrasound patterns, layer or sequence them into complex effects, project them on the user's hand, and export them for use in external programs (e.g., Unity). To create the tool, we interviewed 13 designers who had from a few months to several years of experience with ultrasound, then derived a set of requirements for supporting ultrasound design. We demonstrate the design power of Feellustrator through example applications and an evaluation with 15 participants. Then, we outline future directions for ultrasound haptic design.