go to your zede lift the bonnet and measure the gap between the plenum and the strut bar, thats the basic gap you have to play with.
next get a big ball of bluetack stick it on top of the middle of the strut. now shut the bonnet, it should crush the bluetack ball.
lift the bonnet and, measure the ball. this is the distance between the strut and the bonnet and tells you how much bigger you can make you spacers.
adding the gap space and the bluetack size togehter will give you the absolute max that you can make the spacers, i personally would make it a bit smaller to stop any rubbing.
from memory i would say 10mm should be fine.