I just got my Urban Trail Pack from Alpine Outfitters, it’s made to fit the Alpine Outfitters Adjustable Urban Trail Harness I already had! Here are some photos (I think I make a splendid model):

I haven’t had a chance to trail test it yet, but I think it will work quite well for carrying some of my favorite snacks and maybe a toy or two!
You can check out the Trail Pack on Alpine Outfitters website here (link opens in new tab/window):
http://www.alpineoutfitters.net/scripts/prodView.asp?idproduct=278