ASP.NET MVC: StyleBundle IncludeDirectory & CssRewriteUrlTransform

I have the same problem as many that my images are not resolved when I bundle my CSS-Stylesheets.

Now I found some SO-Answers that suggest using new CssRewriteUrlTransform() as second parameter for the "Include"-Method of a new StyleBundle-Object. This one for example

I'm using IncludeDirectory because I have a directory where I can add files without having them to register somewhere (and they are many files which I don't want to list). But IncludeDirectory doesn't have an override to pass a CssRewriteUrlTransform-Object:

Doesn't work: bundles.Add(new StyleBundle("~/bundles/css/directives").IncludeDirectory("~/app/directives", "*.css", true));

I also tried:

StyleBundle sb = new StyleBundle();
sb.Transforms.Add(new CssRewriteUrlTransform());

But Transforms are of Type IBundleTransform and I'm trying to pass an IItemTransform.

Any suggestions on how to do this?

As I didn't find a better answer, I'll post my solution (which feels more like a workaround):

public class BundleConfig {
    private class CssRewriteUrlTransformWrapper : IItemTransform {
        public string Process(string includedVirtualPath, string input) {
            //see https://stackoverflow.com/questions/19765238/cssrewriteurltransform-with-or-without-virtual-directory
            return new CssRewriteUrlTransform().Process("~" + VirtualPathUtility.ToAbsolute(includedVirtualPath), input);
        }
    }

    private static string MakeVirtualPath(string fromPhysPath, string withVirtualPath) {
        var res = fromPhysPath.Replace("\\", "/");
        var idx = res.IndexOf(withVirtualPath.Replace("~", ""));
        res = "~" + res.Substring(idx);

        return res;
    }

    private static StyleBundle CreateStyleBundleForDir(string virtualPath) {
        StyleBundle res = new StyleBundle(virtualPath + "/bundle");

        string[] cssFilesPhysical = Directory.GetFiles(HttpContext.Current.Server.MapPath(virtualPath), "*.css", SearchOption.AllDirectories);

        List<string> cssFilesVirtual = new List<string>();
        foreach (var file in cssFilesPhysical) {
            res.Include(MakeVirtualPath(file, virtualPath), new CssRewriteUrlTransformWrapper());
        }

        return res;
    }

    public static void RegisterBundles(BundleCollection bundles) {
        bundles.Add(CreateStyleBundleForDir("~/app/custom"));
    }
}

I'm open for constructive citicism :)