Fluid User Interfaces (Fl.UIs) are liquid-based touch surfaces that use computer-vision to detect and interpret a range of tactile user inputs. While Fl.UIs have less input resolution than digital touch screens, they provide an excellent low-cost solution for rapidly prototyping non-rectilinear screen designs as well as exploring novel surface interaction techniques. Fabricated on a laser cutter using low-cost materials, Fl.UIs use unique shape outlines to displace an internal colored liquid to regions-of-interest for a camera. This paper presents a set of software tools that help users rapidly design, fabricate and author interactions with Fl.UIs. The robust construction and an unpowered surface makes Fl.UIs well-suited for outdoor and public installations. The Fl.UIs prototyping tool encourages these uncommon "screens" emerging in complex environments (i.e. urban spaces, benches, tables, fountains, sidewalks).