Riser for Vortex Cantilever mount

[RielM]

I have a Strike Eagle mounted to my AR with a Vortex Cantilever Mount. I'm finding that I'm gravitating towards holding the stock fairly high on my shoulder so that I can get a fast sight picture. I'd like to get the bore axis lower into my shoulder and still get fast target acquisition. Does anyone have any suggestions for a decent quality riser to go between the rail and mount? Ideally something that isn't going to become the weak point in my optic mount and is cheaper than buying a quality higher cantilever mount.