I know what you mean, I don't like it riding inside free either, that's why I strapped it to the seat belt tower, but I would prefer to have it mounted outside, somewhere out of the muck. I can't really mount mine to the front bumper due to the shape of my bumper plus all the stuff in the way, ie: winch, lights, D-rings, etc... I'm in the process of creating my rear bumper, but don't really like the idea of having it mounted across the rear bumper due to the amount of mud it would collect there.
I had it strapped to the spare tire vertically with a couple of mountain bike toe straps, but they didn't hold up to off road abuse. After I get my lift and tires on, as well as the matching-size spare, I was thinking about making a mount on the spare holder somehow...