I put mine in a box big enough to accommodate the base and used 9x13" cake boards (or other cardboard pieces) to raise the sides then tent it all with plastic wrap and tape that in place. Oh, I ALWAYS use a piece of the non skid rubber matting under to prevent shifting. Set it on a completely flat surface in the car.
I just did one of these and of course no actual Doll Cake box to be found where I am. So I purchased a 1/2 sheet box (my base was rather large for the b-day message) then reinforced the corners with super tape and put a small piece of that textured shelf liner in the box and the cake on top. I then put the arms straight up and used them as the center stake for a tent of plastic wrap. I didn't have that far to go but it kept any dust or airborne goodies from landing in my BC. AND of course more shelf liner on the floor of the van to keep the box from sliding about.
I've heard some stores that do the doll cakes will sell you one of their boxes but I had to be inventive.
How to Safely Transport Cakes:
http://forum.cakecentral.com/cake-decorating-ftopict-214391-.html
http://forum.cakecentral.com/cake-decorating-ftopict-214281-.html
HTH
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