Wait, are you talking about movement when you connect a JAMMA harness? The way I read it you were getting movement when you tried to install the board on the holder. Then, yes, there will be some horizontal movement. The goal was to allow different sized boards, if you set up your feet as you've done, you are limited to boards that size. It's certainly possible to put a second sideways slider in there, if that's something you'd want to do.
But making the top slider more aggressive with its grip might damage the boards, so I don't really see a way to do that.
Also, the rails look low to me, if you get any larger PCBs you won't be able to mount them without re-installing them in a higher position. Just FYI. ^__^