Mechanical metamaterials (MMs) exhibit unique properties by manipulating the geometry of individual cells and their overall arrangement into structures. Historically, most MMs are generally fabricated as continuous, anisotropic structures with additive manufacturing, which offers high precision and efficiency but is inherently limited by process and machine constraints. To address this issue, inspired by traditional Chinese mortise and tenon, a novel construction system for plastically isotropic MMs based on customized discrete assembly of a finite set of slender tubular parts is proposed. Discrete assembled thin-walled mechanical metamaterials (DATWMMs) can be spatially constructed for exotic mechanical properties such as isotropic plastic crushing robustness and omnidirectional self-locking capacity. In addition, the construction system endows DATWMMs with tunable plastic energy absorption through the customized assembly of constituent parts with different sizes, sectional configurations, and parent materials. This paper adopts a combined experimental and computational method to investigate the static/dynamic mechanical behaviors and deformation mechanisms of DATWMMs. DATWMMs can outperform comparable modular MMs in terms of specific energy absorption by up to 3600%. Overall, this construction system leverages an incremental assembly process to overcome manufacturing machine-related scale limitations, eliminate printing process-induced material anisotropy, and achieve interchangeability through a consistent assembly process across part types.