LOFT, the Large Observatory For X-ray Timing, has recently pass the ESA call as one of the four M3 mission candidates that will complete for a launch opportunity at the start of the 2020s. The primary goal of the mission is to study strongfield gravity, black hole masses and spins, and the equation of state of ultra-dense matter via high-time-resolution X-ray observations of compact objects. Two instruments will be comprised in the scientific payload of LOFT: the Large Area Detector (LAD) a 12 m2 collimated Xray detector in the 2-30 keV range, with unprecedented timing capabilities, and the Wide Field Monitor (WFM), a coded-mask wide field X-ray monitor. Both instruments are based on silicon drift detectors. The main scope of WFM is to catch good triggering sources to be pointed with LAD, but it will also have its own science programme. The Pre-prototype development of the Detector Tray creating the 3D digital model will need to meet the following requirements: - Alignment of 4 Silicon Drift Detectors (SDDs) with 4 degrees of freedom forming the flat detector plane and its alignment with coded mask with high precision; - Maintain the geometry within working temperatures from -30°C to -20°. Select the materials with low CTE (Coefficient of Thermal Expansion) and with high Heat Transfer Coefficient (HTC) to be used. Mechanical and thermal computing simulations will be performed in order to verify the fulfillment of the requirements.