You should really get something taller to stand on. If she decides to get out from under you, you want a little more height to be able to come down on and something a little more sturdy. Are you doing any ground work with her?
The one thing I've seen that is very different with drafts, you've really got to earn their trust. So, just don't rush it. Take it easy with the backing until she knows you. And definitely get something taller.
I use a 55 gallon drum for my 18 hand Percheron and even that isn't very steady! I've fallen off it twice!