Get a medium size box (approx 10"x10"x10") and place a non-terry (claws will get caught in loops of terry) towel or blanket, put in a piece of branch for her to hold onto (she will feel safer) and then close the box and tape it (just a strip, not like you're sending it to the north pole! lol!) and she will probably sleep and look all bleary eyed when you open it. The box should be big enough to accomodate her with some room to move but not so much that she slides everytime you turn a corner
I see you live in CA but depending on temps you can lay a warm pak (or hand warmer you get at stores that have camping equip) b/t the towel and box, not against her.
She will be fine, just keep the box shut b/c if she sees light she will want to come out. Let us know how you make out!