Rotate SignatureFieldAppearance

Question

I'm trying to add a custom signature appearance in a PDF with iText7, now I want to rotate the whole signature (90 degrees, to put it in the lateral margin of the document). my code is:

PdfReader reader = new PdfReader(inputPDF);
using (FileStream os = new FileStream(outputPDF, FileMode.Create))
{
    PdfSigner signer = new PdfSigner(reader, os, new StampingProperties());

    StampingProperties properties = new StampingProperties();


    SignatureFieldAppearance sfa = new SignatureFieldAppearance("TestSignature");
    
    signer.SetSignatureCreator("Me");
    signer.SetSignatureAppearance(sfa);
    
    signer.SetReason(!string.IsNullOrEmpty(SigReason) ? SigReason.ToUpper() : string.Empty);

    signer.SetContact(contact);
    signer.SetLocation(!string.IsNullOrEmpty(SigLocation) ? SigLocation.ToUpper() : string.Empty);

    StringBuilder sb = new StringBuilder();


    sb.AppendLine("Documento firmato digitalmente da " + name.ToUpper());
    sb.AppendLine(now.ToString("dddd dd/MM/yyyy").ToUpper() + " alle " + now.ToString("HH:mm:ss"));
    if (!string.IsNullOrEmpty(SigReason))
        sb.AppendLine("Motivo della firma: " + SigReason.ToUpper());
    if (!string.IsNullOrEmpty(SigLocation))
        sb.AppendLine("Luogo Firma: " + SigLocation.ToUpper());

   
    var assembly = typeof(PaDESSigner).GetTypeInfo().Assembly;
    byte[] imgBytes = new byte[0];
    using (var stream = new MemoryStream())
    {
        Resources.pdf_signatureLogo.Save(stream, System.Drawing.Imaging.ImageFormat.Jpeg);
        imgBytes = stream.ToArray();
    }
    iText.IO.Image.ImageData img = iText.IO.Image.ImageDataFactory.Create(imgBytes);
    img.SetXYRatio(0.5f);
    img.SetRotation(angle);
    sfa.SetContent(sb.ToString(), img);

    AffineTransform af = new AffineTransform();
    af.Rotate(Math.PI / 180 * angle);

    iText.Kernel.Geom.Rectangle rectangle = new iText.Kernel.Geom.Rectangle(x, y, w, h);
    
    signer.SetPageRect(rectangle);
    signer.SetPageNumber(_page);
    PdfFormXObject foregroundLayer = new PdfFormXObject(rectangle);
    
    PdfCanvas canvas = new PdfCanvas(foregroundLayer, signer.GetDocument());
    
    canvas.ConcatMatrix(af);
    
    canvas.SaveState();
    signer.SetSignDate(now);

    signer.GetSignatureField().SetSignatureAppearanceLayer(foregroundLayer);
    signer.GetDocument().GetCatalog().SetModified();

    X509Certificate2Signature externalSignature = new X509Certificate2Signature(myCert, "SHA256");
    signer.SignDetached(externalSignature, chain.ToArray(), null, null, null, 0, PdfSigner.CryptoStandard.CMS);

    try
    {
        File.Delete(System.IO.Path.GetDirectoryName(Assembly.GetEntryAssembly().Location) + System.IO.Path.DirectorySeparatorChar + "Temp" + System.IO.Path.DirectorySeparatorChar + "t0.pdf");
    }
    catch { }
    try
    {
        File.Delete(System.IO.Path.GetDirectoryName(Assembly.GetEntryAssembly().Location) + System.IO.Path.DirectorySeparatorChar + "Temp" + System.IO.Path.DirectorySeparatorChar + "t2.pdf");
    }
    catch { }

    return true;
}

Now, if I don't use the canvas and "foregroundLayer" everything works except the signature doesn't - obviously - rotate.

With my actual code the pdf is signed, but the signature field is invisible, or, I think, goes on a layer under the actual visible document.

How can I remedy? I think I have to "associate" the canvas to a layer on top, but I don't know how, nor if it's the right path.