Where are you taking her too? Really hope it all goes well. A number of people have said to me that on longer journeys their buns have settled because they have time too, and ultimately travelled really well. I spoke to a good bunny friend last night and mentioned your thread, and she said she had been going to make the mistake of travelling hers in a large cage so that they had lots of space, but that is really the worst thing you could do, as they would be sliding all over the place and probably be just terrified. My bunnies are all perfectly fine in carriers that are big enough for them to turn round that kind of thing, and space to properly stretch out if they want, but they seem comforted and secure by the 4 relatively close walls if you know what I mean, and they can't slide about.
I'm sure you wouldn't and I hope you don't mind me saying, but I have seen carriers just put on the floor or back seat, which is terribly dangerous as well, and I put the seat belt over one side of the carrier and then use one of those bungee clip things are they called, do you know what I mean, hook each end and stretchy in the middle, and then I use one of those to secure like the seat belt but coming from the other side, so in effect they have a big X across the front of their carrier