[Inkscape-user] exporting slices as boxes

Hi! I've made some graphics for web pages with inkscape, but I find hard to export certain areas. For example, I have a Box, a rounded corener one, that should be exported in three vertical slices, the middle one, will be repeated as a background latter. The point is I make a rectagle surrounding the area I want to export, then, I lower transparency to 0%, so the box won't be exported, but the drawing inside its area (this box has no stroke). Then when I want to select this "ghost" box, it's gone! If I select an area that contains the box, there it is, but, no simle way to select just _that_ box. Any suggestion? I think that maybe the search box may help, but I don't undesrtand how it works. Thanks in advance. Nahuel