Guys,
Try this trick,
I use it on all my Aussies, and other fur paws, to get used to the trailer, and riding,
move their bed, food and water dish inside the trailer, don't wimp out and give in,
after a day or so, they'll like their new home, and won't want to give it up.
To get use to the trailer movement, with that type of halter you have on your fur baby, get a quick snap and a couple pieces of chain lengths, an "eye bolt",
and towards the front find the center and mount the "eye bolt" with HD washers on each side.
Open the "eye bolt" end just enough to slip the end of the chain loop thru it.
You'll close up the "Eye bolt after the correct distance is found).
Then have them stand right over the eye bolt and adjust the chain links with a quick snap attached to the bottom ring of the halter, make sure you take the slack out of the halter when measuring, you want a firm support on the halter, around and holding them in that place.
Then snap them in and get use to the feeling of being tethered to that spot,
they may fight it, but if they can't get their front paws up and over the side rails,
they can't bail out.
Leave them there for a short time and moved the trailer around with them tethered there. After a short while they will get used to and like it, then start making short rides around the yard to get her used to the sounds/movements of the trailer etc.
My Aussie and German shepherd love riding all the toys I have including the ATV/SxS, they soon learn that the tethered helps them with any quick movement of the ride, and lean into the tether for support etc.
You can also use a wheelbarrow to start training them, with the same principal with the snap chain, for smaller paws etc.
Hth's