Susan, a friend a few years ago had similar problems. He had a gelding who
would not travel, so he packed a feed bag full of straw and wedged it between
the horse and the outside wall of the trailer. Worked a charm, as the horse
had something to brace against. I've also found, in the past, that mares tend
to want more room to spread their back legs than geldings, however, I have
experienced exceptions to the rule.