Likely the OS gurus will have suggestions, but depending on your requirements for building geometry you could use the following public domain tools/methods.

• Use CONTAM to create scaled, rectilinear shaped buildings (this is the main limitation on geometry). Drawing/defining multilevel buildings with ContamW is very simple.
• Use the Contam3DExport tool to extrude them to IDF files. Allows you to create IDF using pre-defined IDFs that contain building constructions/materials to be assigned based on surface type, e.g., exterior walls, interior walls, floors, etc. Can even create AirLoopHVAC for multiple systems if desired.
• Use eppy to modify IDF files as desired.

Let me know if you need more info.

Stuart