I'm not sure I'm understanding you on this. Is it too far behind the wall, or are the 2 2x4s are too far from each other. If they're too far from each other you can just put a few a few 2x4's going across. The picture should give you a good idea-

If that's out of the question you can always add it do a fixed object like pole used for a basketball hoop, or a tree. If there's nothing that you can find you can pay a bunch of money and get a freestanding speedbag platform.