Ev, just grab the main leaf (with the eyes) Cut the eyes off and place it under the main leaf of the pack your using. If thats not enough, grab the 2nd leaf, cut it in a way so that it ends half way between the 2nd and 3rd leaf.
Just be cautious of how much thread you have left on the center bolt. mightnt be able to add a 2nd extra leaf.
We deal with half traction (anti tramp) leaves at work.
Following on from what Andy43 said, thickening only the front half of the leaf pack, produces an almost 2 link style setup.
The front half acts as a control arm whilst the rear is providing the suspension.
Stock leaves are 6-7mm thick. I will be going new leaf packs, reversed eyes, extra leaf and an 8mm half traction leaf in the wagon.
Just some food for thought for you guys playing with leaf sprung rear ends.