xlsgen 4.0 adds support for creating XLSB files.

XLSB (B is for binary) files are a variant of XLSX files, in that it uses binary records internally instead of XML, for the most part.

Support for reading XLSB files was added in xlsgen 3.5. It is the writing that is new in xlsgen 4.0

Of course, this is a comprehensive feature which includes cells, formatting, formulas, charts and so on.