I found that I still had to exert a lot of pressure down, as I drove in the screws, to keep the pieces from steady. I generally clamp both pieces down to the table when driving pocket hole screws, so this was something I hasn't foreseen. I suppose I could've clamped them down to the table, after clamping them together.
In any case, I think you could build your own with little effort/time and make it better than the Rockler version.