border-image overflowing % based grid system

I'm using a grid system that uses percents (the 1140px grid to be precise) and trying to use CSS3 border-images on it. It all works correctly, but I'm using some fairly wide fancy borders which are throwing off the width of the box. Is the开发者_StackOverflow中文版re any way to fix this? (or to do a way to do some fancy css borders without the CSS3? I'd love that, but can't find how and am failing at the moment.)

If you add the following to the box's CSS

box-sizing:border-box;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;

Then it will add bordes inside the defined width rather than outside.