The DarkSide-20k experiment is an international pioneering scientific initiative aimed at the direct detection of dark matter through a dual-phase Liquid Argon Time Projection Chamber (TPC) operating at cryogenic temperatures of 87 K (-186 °C).The experiment is being constructed within the Laboratori Nazionali del Gran Sasso (LNGS) of the Istituto Nazionale di Fisica Nucleare (INFN), a unique research facility due to its unconventional location. The experimental apparatuses are installed in an underground space excavated beneath the central part of the Gran Sasso mountain range, shielded by approximately 1,400 meters of rock, significantly reducing the flux of cosmic rays and making the site ideal for rare-event physics experiments.The TPC will be housed within a stainless-steel vessel, which is itself contained inside a sophisticated cryostat structure. The total weight of the TPC enclosed in the vessel is approximately 30 tons.The cryostat features an innovative design, incorporating IPE600V steel beams and a ribbed tertiary steel membrane with structural functions, along with a multi-layered secondary and primary membrane system providing essential thermal insulation. In the final configuration, the vessel will be suspended from the cryostat roof.The installation of such structural components in an underground cryogenic environment posed several engineering challenges, requiring tailored structural solutions.To complete the installation of the detector components within the cryostat, it was necessary to construct a cleanroom directly above the cryostat roof. This cleanroom is essential for assembling all TPC components in a controlled environment, targeting ISO 7 (ideally ISO 6) cleanliness standards.The structural elements supporting the cleanroom had to fulfill multiple engineering roles and conform to stringent construction constraints. This led to the design of a non-conventional structure subject to numerous geometric limitations, and tailored to fit the cryostat configuration and available access paths—all while ensuring mechanical performance, modularity, and radiopurity.In particular, for the installation phase, a temporary support system inside the cryostat was also required. Since the primary membrane was designed to withstand uniform liquid argon pressure and not localized mechanical loads, a new structural flooring system—referred to as the "false floor"—was developed.This system had to uniformly distribute loads, minimize deflections to prevent contact with membrane ribs, meet cleanroom standards, and completely removable in small pieces without any on-site cutting.The adopted solution was a composite modular system composed of carbon steel lattice beams, circular-base support rods, high-strength gratings, and a stainless-steel finishing layer. The platform was designed to resist concentrated loads from personnel and equipment while preserving the cryostat’s structural and radiological integrity.Advanced Finite Element Analyses (FEA) were carried out to verify structural performance under service conditions, ensuring that stresses remained below material yield limits and deflections within acceptable tolerances. A simplified modal analysis was also performed to assess the dynamic response, comparing the resulting frequencies with the site-specific seismic spectrum, in compliance with Italian seismic regulations.An experimental load test campaign further validated the numerical predictions and confirmed the platform’s reliability for on-site operations.This work presents an integrated engineering case study, combining structural design, numerical modelling, and on-site implementation under extreme environmental and operational constraints. The methodology and solutions adopted can serve as a reference for future projects requiring high-performance temporary structural systems in cryogenic and cleanroom environments.