4. Press [command] A to select all the objects, and then press [command] / to bring up the Align dialogue box. Click on Center in both the Horizontal and Depth axes, and then click OK. Now use the Filled Polygon tool from the Tool palette to draw an octagonal shape around the cylinders as above. Double-click it to take it into the 2D Sculptor modeller and clean up the points. Click OK when you're happy.