Questions about building a small rat rack!

I've been having difficulty finding a rat rack plan for a four-tub rack, and I was also wondering about the size I need. I only want to have a 1.2 ratio per tub, do I need to go the concrete mixing tub route, or can I go smaller with cat litter boxes?

You could go smaller, but the difference between a large litter box and the mixing tub is not much. Go with the tubs. That way if you decide you need more rats, you're already prepared. There are a couple of "how to" threads here and over on fauna. With a few tools you can make on pretty easily.