Please try and convince her to allow your horse to travel loose. If its her first few times trailering she still needs to find her balance. Tying her will probably not help her with finding her balance.
My friend had a horse that was very well trained, tied well, trailered well, not spooky at all, but something spooked him in the trailer and he freaked out and hung himself to death.
At fair practice last year someone tied their horse who wasn't the world's most respectful horse and they had a small trailer with no matts in it. The horse freaked out, fell in the trailer, and she was in such an awkward position that she couldn't get up. She tried once and kicked a window out, tried again and kicked another window out. Finally someone with common sense helped the horse with getting a rope around her legs and moving them to help her. They finally got her up and out, she damaged her leg pretty good.
My show horse last year I always tied because I only had a stock trailer with no dividers and I put my tack in the front and I didn't want her to nose it and knock it over, but then again she is the type of horse that falls asleep the moment she walks into the trailer. Not kidding.
When we first got our Thoroughbred mare we took up a friends 4-horse slant show trailer because we didn't have a trailer. We tied all the slants back and kept her loose the 5 hour ride home. She is a very claustophobic horse, we have a stock trailer which helps her a lot. But we tie her sometimes if she starts spinning in the trailer.
These are just my experiences, but let your horse learn its balance in the trailer, then worry about your slants. Horses DO NOT like to be in small spaces.