I used the screw it came with. If you look at the pic you can see I tore up the screw head pretty bad. That screw was a pain to get out and I should have bought a few replacements. You can get them from Cz USA.
It think it was a 3mm setup, might have been 4mm. The cool thing about going this route is you can adjust the pad to the angle that works best for your thumb. You will need to loctite it in when you figure out where you want it.