[Unofornaio]

Newspaper, broken pieces of block, mortar slobbers, concrete slobbers, tar paper, paper grocery bags, also work just fine..
If you are using redi mix the slump shouldn't be that high anyway as to "flow" through any of these. If you are mixing by hand use the bags and with the first batch you mix top off the webs so it sets up a bit while you are mixing the rest of the slab.
Keep in mind the wetter you make concrete the weaker it gets and the more shrinkage/cracks you get.