How to create an excel / XML file in memory within a plugin?

Hello everyone,

I am trying to create a new excel file and attatch it as an attachment in a note within a plugin. I am facing the following issues.

1.- If I use the Microsoft.Office.Interop.Excel library, I cannot save the file on a folder since I am within a plugin, so I cannot access the drive.

2.- I cannot retrieve the XML data from an Interop object to convert from XML to bytes and from bytes to 64string and add the attachment to the body.

3.- If I use OpenXML library, I cannot create a new file in memory, I just can read an existing file and store it on a memory stream.

I need to be able to create a new excel document, fill some information in it and add it as an XMLfile to the documentbody of an annotation entity.

Does someone realize something like this before and can give me any advices?

Thank you very much  in advance!