In this paper, we explore quick 3D shape composition during early-phase spatial design ideation. Our approach is to re-purpose a smartphone as a hand-held reference plane for creating, modifying, and manipulating 3D sweep surfaces. We implemented MobiSweep, a prototype application to explore a new design space of constrained spatial interactions that combine direct orientation control with indirect position control via well-established multi-touch gestures. MobiSweep leverages kinesthetically aware interactions for the creation of a sweep surface without explicit position tracking. The design concepts generated by users, in conjunction with their feedback, demonstrate the potential of such interactions in enabling spatial ideation.