Just use a cardboard box. Go to FedEx or UPS and they have a bunch of different sizes. If you have a 12" round, I'm guessing you will put it on a 14 or 16" board, so you'll need to look for a box that's big enough to accommodate that. The easiest thing to do is to tape up one end of the box, then set it on it's side and slide the cake in that way. Then just tape up the open end. That means that you need a box that's at least 16" high, by whatever the height of your finished cake. If the box ends up being a little too big, you can either hot-glue some pieces of foam around the base to keep it centered in the box, or put a piece of double-sided sticky tape under the edge of the base after you get it in the box. Make sense?
When you stack your tiers, just use a cardboard cake round between them (rather than plastic plates), and hammer a sharpened dowel rod down through from the top. That way you can move it anywhere you want without fear of the top tier sliding off. Be sure to use drinking straws in the base tier to sit the top tier on. Feel free to send me a PM if this isn't making sense.