It needs to be securely attached to the tow vehicle (and separately to the tow hitch). If it's over the tow bar, in an emergency it will just slip off over the ball and follow the detached trailer down the hill.
I've got one who Amigo's fit like a glove, and one (built like a bulldog) who Amigos most definitely do not fit at all and who seems to only fit into a Rhino as they are built for big shoulders/necks. If you are happy with how your Amigos fit, I would stick with Amigos.
Just off the top of my head, I know that both BD and BS accept ANY make of treeless as long as it resembles a traditional tree'd saddle (ie, not a western type). Not sure about BE as we aren't members.
Right next to his face, on the wall at the back of the lorry, at a right angle to the window. So he had window infront of him, haynet sat on top of the tack locker, then immediately to his right was the mirror, just as if he had a horse stood right next to him. We have since sold the lorry and...
We had the same thing with one of ours. My husband figured that the pony just didn't want to travel alone and would like a friend. So in a moment of madness, he decided to order a large, acrylic safety mirror (which I scoffed at as I was convinced it wouldn't work). We stuck it up using...
Drill 4 holes in the outer edge of a plastic dustbin lid and hang it upside down with bailer twine just below the nest. Then bin the whole thing once they have gone.
I always wear it. Even when I come off the (tiny stretch of) road and onto the thousands of acres of off-road riding we have on our doorstep, because I want the air ambulance to be able to find me if I ever need it!
If you long rein she wont lose any of the muscle she has built up and may well build up more. You wont need to do added exercise as long reining her up hill and down dale will have you fitter than a flea. Win, win.