I'm going to be pouring a 8.5' x 16' concrete slab. I've dug up the yard and flattened it out fairly well. I built the form out of 2x4's and used 2x2's posts every few feet for anchoring it. In order to make the form level, it has some gaps underneath the 2x4's.
What is the best way to fill in the gaps under the 2x4 form?