I had the same problem of too-low bottle bosses (since I prefer to use short bottles), and could not simply use a clamp-on bracket because the down tube is not round but "streamlined" (yeah, right).
What I did is get a strip of aluminum bar (from a hardware store). Without going out to the garage to look at it, I'm thinking it's about 1" wide and 1/4" thick, or maybe a little smaller than that.
I cut off a piece of the bar that was twice the distance between the bosses, plus about 2". I then drilled three holes in the bar, spaced as far apart as the bosses. (This places the first hole about 1" from one end of the bar, the second in the middle, and the third 1" from the other end.)
To mount, first use a screw through the bottom hole in the bar and into the bottom boss. Next take your cage and run a screw (you may need to get a longer one) through the bottom hole of the cage, the middle hole of the bar, and into the top boss. Finally use a screw and nut to secure the top of the cage to the bar, through the top hole.
I've found this to be quite stable -- no problem with rattling, etc (though I suppose it might be a bit rattly for mountain bikers), and it has never bent or come loose.