Notification Window - Preventing the window from ever getting focus

I'm having some issues getting a notification box to behave correctly in c#. Basically I'm showing a boarderless form in the lower right hand side of the screen, which displays a message for a few seconds and then disappears. The problem is that I need it to appear on top of other windows without it ever being able to steal focus. Ideally, I want it to be purely managed code, although looking through similar examples I doubt this will be possible.

At the moment I'm preventing it from stealing focus when calling Form.Show() with an override:

protected override bool ShowWithoutActivation // stops the window from stealing focus
{
    get { return true; }
}

and then ignoring mouse clicks with:

    private const int WM_MOUSEACTIVATE = 0x0021;
    private const int MA_NOACTIVATEANDEAT = 0x0004;

    protected override void WndProc(ref Message m)
    {
        if (m.Msg == WM_MOUSEACTIVATE)
        {
            m.Result = (IntPtr)MA_NOACTIVATEANDEAT;
            return;
        }
        base.WndProc(ref m);
    }

However I find that if I use these in conjunction with TopMost = true (which I need), it gains focus anyway, and if all other windows are minimised, it also gains focus.

So, is there any way to flat out prevent a form from ever gaining focus (whether via mouse click, alt-tab, etc), while still being the top most/second top most form? Even just giving focus immediately back to the window it stole it from would work (although introduce flickering).

Any suggestions would be greatly appreciated, I'm really stuck with this.

EDIT:

Ok, so I finally managed to get this working using:

protected override bool ShowWithoutActivation // stops the window from stealing focus
{
    get { return true; }
}

// and

const int WS_EX_NOACTIVATE = 0x08000000;
const int WS_EX_TOPMOST = 0x00000008;

protected override CreateParams CreateParams
{
    get
    {