Three-dimensional mesostructures enrich coarse macrosurfaces with complex features, which are 3D geometry with arbitrary topology in essence, but are expected to be self-similar with no tiling artifacts, just like texture-based material models. This is a challenging task, as no existing modeling tool provides the right constraints in the design phase to ensure such properties while maintaining real-time editing capabilities. In this paper, we propose MesoGen, a novel tile-centric authoring approach for the design of procedural mesostructures featuring non-periodic self-similarity while being represented as a compact and GPU-friendly model. We ensure by construction the continuity of the mesostructure: the user designs a set of atomic tiles by drawing 2D cross-sections on the interfaces between tiles, and selecting pairs of cross-sections to be connected as strands, i.e., 3D sweep surfaces. In parallel, a tiling engine continuously fills the shell space of the macrosurface with the so-defined tile set while ensuring that only matching interfaces are in contact. Moreover, the engine suggests to the user the addition of new tiles whenever the problem happens to be over-constrained. As a result, our method allows for the rapid creation of complex, seamless procedural mesostructure and is particularly adapted for wicker-like ones, often impossible to achieve with scattering-based mesostructure synthesis methods.