Use small terra cotta pots, but contain them with a rectangular plastic tray (a sort of window box tray). Still, have to warn you, when there is soil involved, there is always a
danger of spilling. Murphy lives on
boats with us. However, the general shape can be secured on a counter, or under the
dodger.
Just another comment, you may be happy with only growing herbs, which tend to keep away bugs, but you'd be amazed to find that aphids and other pests WILL fly out and infest your plants. Sorry, but it's the way of the bug. If you keep close watch, you may be able to pick them all off, but if you don't, the spiders will follow the flying insects, and they'll stay as long as there's
food for them (the insects).
Good luck with it, 'cause fresh herbs are lovely!
Ann