Coreless electric motors are gaining interest from the community as they have a larger power-to-weight ratio and higher efficiency. The absence of iron slots leaves the winding space unobstructed. It enables the geometry of the coils to be skewed to overlap with their neighbors. This work proposes a topology optimization method to design 3D-printed motor windings aiming for coreless electrical machines with higher motor-constant. A multi-coil interpolation scheme is proposed to design a single coil while accounting for packing with the other ones constituting the winding, without requiring assumptions on the shape or position of the coil. The standard 2-D topology optimization is extended to a 2.5-D one so as to provide more freedom on the achievable geometries while keeping a reasonable computational time. The developed computational design tool generated a novel motor winding with high performance.