Mocking OpenXML SDK SpreadsheetDocument

I would like to use process described here: Automated Testing OpenXML SDK

However, what does it take to mock the something like this? I have made the following interface:

public interface IExcelDocument
{
    Row GetRow(uint rowIndex, SheetData sheetData);
    SharedStringTablePart GetSharedStringTablePart(SpreadsheetDocument excelDoc);
    WorksheetPart GetWorksheetPart(SpreadsheetDocument excelDoc, string sheetName);
    Cell InsertCellInWorksheet(string columnName, uint rowIndex, WorksheetPart worksheetPart);
    Row InsertRow(WorksheetPart worksheetPart);
    int InsertSharedStringItem(string text, SharedStringTablePart shareStringPart);
}

I would imagine mocking would look something like this:

[TestMethod()]
public void Excel_GetWorkseetPartTest()
{
    Mock<IExcelDocument> mockExcelDocument = new Mock<IExcelDocument>();
    string sheetName = "sheet";
    var excelMock = mockExcelDocument.Object.GetWorksheetPart(MySpreadsheetDocument, sheetName);

    Assert.IsTrue(excelMock != null);
}

GetWorksheetPart method I want to unit test looks like this:

public WorksheetPart GetWorksheetPart(SpreadsheetDocument excelDoc, string sheetName)
{
    Sheet sheet = excelDoc.WorkbookPart.Workbook.Descendants<Sheet>()
        .SingleOrDefault(s => s.Name == sheetName);
    if (sheet == null)
    {
        throw new ArgumentException(
            String.Format("No sheet named {0} found in spreadsheet {1}",
                sheetName, _filePath), "sheetName");
    }
    return (WorksheetPart)excelDoc.WorkbookPart.GetPartById(sheet.Id);
}

I am not able to wrap around MySpreadsheetDocument because I would need to also implement the SpreadsheetDocument.Open method and not sure even if that is reasonable.

Here is how I call GetWorksheetPart:

using (SpreadsheetDocument _excelDoc = SpreadsheetDocument.Open(_filePath, true))
{
    IExcelDocument excelDoc = new ExcelDocument();
    WorksheetPart worksheetPart = excelDoc.GetWorksheetPart(_excelDoc, sheetName);
}