My driver's side mat keeps moving about as the top plugs holding the mat are gone. I have one of the caps I found in the car, but it does not have a screw like the other three mats.

Is the drivers side different?

Ok I had a really good look at them. It turns out the plug I had was not for the carpet mats.
It appears the front mats have a screw/push type cap with a threaded section in the centre, but the rear mats have a screw hole in the centre which holds the top cap to the bottom part.