Labs_underpass_p.upk Here

It allows for storing assets inside your level package, essential for functioning with the Rocket League Workshop.

It is considered one of the best methods to load custom maps on PC. How to Set Up Custom Maps (Step-by-Step) Labs_Underpass_P.upk

Disclaimer: Modding Rocket League is done at your own risk. Always backup your files. Map files & packages - RL Modding Wiki It allows for storing assets inside your level

You can directly create or import meshes and textures inside your level package. Labs_Underpass_P.upk

Open the game, select "Freeplay" (or local match) and choose "Underpass". Your custom map will now load. Pro Tips for Map Modding

For easier management, consider using the Rocket League Map Manager on GitHub to switch between custom maps automatically.