I just use a trailer tie that has a panic snap on one end (which I will attach near the window) and a normal snap on the other (that I will attach to the horse).
I also trailer my horses with breakaway halters. Just in case they would ever fall down in the trailer, hopefully the halter would break so that they aren't hanging themselves by the tie.
Which .... these trailer ties would accomplish! I have never used them personally, but it has the two things I want when tieing in a trailer: panic snaps and breakaway.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.