How to implement a custom Windows::Storage buffer with C++/WinRT

buffer.cpp

struct __declspec(uuid("905a0fef-bc53-11df-8c49-001e4fc686da")) IBufferByteAccess : ::IUnknown
{
    virtual HRESULT __stdcall Buffer(uint8_t** value) = 0;
};

struct CustomBuffer : implements<CustomBuffer, IBuffer, IBufferByteAccess>
{
    std::vector<uint8_t> m_buffer;
    uint32_t m_length{};

    CustomBuffer(uint32_t size) :
        m_buffer(size)
    {
    }

    uint32_t Capacity() const
    {
        return m_buffer.size();
    }

    uint32_t Length() const
    {
        return m_length;
    }

    void Length(uint32_t value)
    {
        if (value > m_buffer.size())
        {
            throw hresult_invalid_argument();
        }

        m_length = value;
    }

    HRESULT __stdcall Buffer(uint8_t** value) final
    {
        *value = m_buffer.data();
        return S_OK;
    }
};

IAsyncAction SampleUsage()
{
    FileOpenPicker picker;
    picker.FileTypeFilter().Append(L".txt");
    picker.SuggestedStartLocation(PickerLocationId::PicturesLibrary);
    StorageFile file = co_await picker.PickSingleFileAsync();

    if (file == nullptr)
    {
        co_return;
    }

    IRandomAccessStream stream = co_await file.OpenAsync(FileAccessMode::Read);

    IBuffer buffer = make<CustomBuffer>(100);
    co_await stream.ReadAsync(buffer, buffer.Capacity(), InputStreamOptions::None);

    com_ptr<IBufferByteAccess> byte_access = buffer.as<IBufferByteAccess>();
    uint8_t* bytes{};
    byte_access->Buffer(&bytes);

    WINRT_TRACE("%.*s", buffer.Length(), bytes);
}

Hello,
I seem to get C++ nonvirtual function cannot be declared with 'final' modifier at following line:

HRESULT __stdcall Buffer(uint8_t** value) final

Removing the final removes the error, but I don't know if that's fine to do.
I have not much experience in C++/WinRT in general, your advice would be great.

Thanks.