1) Put the red builder brush at the center of the map.
2) Use Ctrl-alt to make a box in the top viewport to select all of the meshes. (use ctrl-alt-shift if you need to add more to the selection)
3) Hit Ctrl-W to duplicate them.
4) Hold Ctrl and select the red brush.
5) Click on the small red dot in the middle of the red brush.
6) Hit the mirrorX or mirrorY button.
7) Hold Ctrl and move the red brush up one grid line and to the left one grid line.