You can likely buy metal for less than you can build with wood right now. with the cost of plywood right ow you can spend a lot of cash for no so much wood. The shelf suggested above would work great. You can also get very heavy duty shelves that hold 400lbs/shelf at Sam's Club (used to be called Gorilla Shelves when we bought them) or Big Box Stores for $150 or less.
If you go with wood, I'd use 2x4 common pine bolted together at each junction. Shelves would likely need to be 3/4" plywood (strand board is fine but add supports in longer sections to prevent sagging). Depending on shelf length you're really only looking at 50lbs or so per carboy.