xlslib, at a brief glance, looks like it is based on the .xsl format, not .xlsx. Wondering if it is easier to generate .xlsx XML files for export, or not.
http://msdn.microsoft.com/en-us/library/aa140066%28office.10...
It only describes the data part and doesn't document Excel options such as print settings and filters; if you need them, create a sample in Excel, save it as XML Spreadsheet 2003 and examine the result.
(The 'big' Excel format that is a part of Office Open XML format is also called Spreadsheet ML, but they're different; from what I understand this one is a subset.)