I thought I'd pass this along to everyone: I've discovered that the boxes in which large tubes of toothpaste are packaged serve as pretty good mold-boxes. Slice off one side of the box, re-enforce with a little duct tape, et voila! They are deep enough that you can make molds for plugs, and you can adjust their length with scissors and re-seal with a hot glue gun. Give it a try for smaller tasks.
Cal