Burris Fast Fire II riser/shims?

I recently bought a Burris Fast Fire II red dot sight for my Mossberg 500. When I initiallt installed it, I had no issues.

I have since switched to the Blackhawk Industries (Knoxx) style of recoil supressing stock, and find that my head/eye does not align low enough with the Fast Fire II at it’s lowest setting.

Does anyone know of a way to shim these sights to get my red dot back in line with my eyes?

Love this sight in case anyone is wondering!