CustomViewVirtualPathProvider.cs

    public class CustomViewVirtualPathProvider : VirtualPathProvider
    {
        public override bool FileExists(string virtualPath)
        {
            return File.Exists(HostingEnvironment.MapPath(virtualPath));
        }

        public override VirtualFile GetFile(string virtualPath)
        {
            var customizedVirtualPath = virtualPath.Replace(HostingEnvironment.ApplicationVirtualPath, HostingEnvironment.ApplicationVirtualPath + "/CustomViews/");
            return FileExists(customizedVirtualPath) 
                ? new CustomViewVirtualPath(virtualPath, HostingEnvironment.MapPath(customizedVirtualPath)) 
                : Previous.GetFile(virtualPath);
        }
    }

    public class CustomViewVirtualPath : VirtualFile
    {
        private readonly string _physicalPath;

        public CustomViewVirtualPath(string virtualPath, string physicalPath) : base(virtualPath)
        {
            _physicalPath = physicalPath;
        }

        public override Stream Open()
        {
            return new FileStream(_physicalPath, FileMode.Open, FileAccess.Read, FileShare.Read);
        }
    }