Sorry, I didn't include a plug with the above amphora.  Plugs were commonly made of beeswax, low-fired clay, leather, tar, etc.. depending upon the material being shipped.  So, here's an amphora and a plug to go with it.  (You can drop the plug down into the amphora, give it another color, maybe a bump map and it becomes the "contents" of the amphora in a low-poly background image.

http://img260.imageshack.us/img260/662/amphorawplugnw9.png (pic)

http://www.mediafire.com/?levwxmr1yn3 (Amphora-with-plug.rar file - two separate objects)