Based on where you're located, perhaps check w/ a local grocery store and see if they have any boxes available that are meant for doll cakes.
I got one from a local grocery store, and it is literally shaped like a "doll house" and it has a lift out cardboard insert, and on the cardboard insert are these 4 little pop up things, that will keep a round cake base in place quite nicely.
I've made 4 of these and this always seems to be my worst part! These were not paid cakes but for my nieces. The first time I had a Cavalier and set the cake on the passenger side floor in the front seat and drove hunched over holding Barbie's hand!?!? 
Picture it! That's the only thing I think of now because I think the other times, my husband drove and she sat in my lap! 
Don't know if it helps but maybe you'll get a good chuckle!
Sandi
I have a barbie doll cake to deliver tomorrow. I found by luck that a box that resturants get gallons of items in works perfect or at least it fit perfect. (The one I found was where a resturant had cocktail sauce in. It says 4-1 gal jars.) She fit almost perfect and I feel should travel well.. Hope that helps. My husband does floor care for some resturants, and I noticed that the boxes that cases of eggs come in are about the right size too. Just an idea hope it helps!!!! 
just go slow, slow turns. have it on a non slip mat in a box (if you like, top open) and the box on a non slip mat.
yep, that's what i use too... just didn't know the name for them. foam mats you put in your silverware drawer. you can buy a large roll at costco. i lined all the drawers of my craftsman tool cart (where i keep my stuff) with it too
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