I get by without modules or header files in my C++ projects by using the following guidelines:

- Single translation unit (main.cpp)

- Include all other cpp files in main

- Include files in dependency order (no forward declarations)

- No circular dependencies between files

- Each file has its own namespace (e.g. namespace draw in draw.cpp)

This works well for small to medium sized projects (on the order of 10k lines). I suspect it will scale to 100k-1M line projects as long as there is minimal use of features that kill compile times (e.g. templates).