impossible. the remaining slop has nothing to do with the rack. it is my opinion that all of the other components in the system (the posts, the arms, the ball cups, etc) at least minimally contribute to the slop. there is no more slop with the rack itself.
on a side note, i also got the rear clamping hexes. was disappointed to see it came with crappy little 2mm button head screws. i won't even install these until i get some 2x6 cap heads.