I've never really played with clay slip, but I got a drywall blender (I think this mixer that goes in your drill has a better name; it will come to me later), I got this drywall blender and tried whipping up a batch of slip today.
It's just soil, no amendments.
In about half an hour, they were stuck tightly enough to hold the stack of four (about thirty pounds) by the top brick. I wasn't expecting that. NEAT.
Solar Station Construction Plans by Ben Peterson -- ebook