Well, I wrote a long comment but then decided not to post it. Didn't want to burst your bubble.
So here it is, I think that putting anymore weight on the already overwieght rear door is a bad idea. Unless you don't plan on keeping your H3 long, I predict that your rear door will begin sagging and will require readjustments becoming more and more frequent. I would put my time and effort somewhere else. Additionaly I think it will make access to your roof rack much more difficult. Not to mention the concern you have of pushing the tire out over and inch. Save your money and get a good adventure trailer. My two cents, hope it helps.