Plastic shims might increase aggression levels by increasing the gap, but it wont improve the stiffness & rigidity of the blade. But my main reason for shimming is that increase of stiffness/rigidity.Shims can be made from thin, soft plastic, cut with scissors. You will need to make holes for the posts and the screw. Plastic packaging is ubiquitous and destined to be thrown out, so cheap and abundant. Just be aware that plastic bottles may not have a totally uniform material thickness.