If the model is a thin shell, use the Solidify Modifier to give it wall thickness, making it a "solid" piece rather than a flat plane.
Before modeling, you must ensure the source files are clean and watertight.
Use tools like WinRAR or 7-Zip to extract the contents of the .rar file to your desktop.
How to make a 3D model solid in Lychee when it ... - Facebook