The boxes look great. I would recommend drilling a few small holes in the side close to the bottom. This will allow water to drain out so your plants won't drown without having to worry about weeds growing up through the bottom (we have the worst quack grass here...it can spread roots a foot deep in all directions before popping up - quack grass took over our sandbox even though we covered the bottom with weed barrier
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You can fill the bottom of your boxes with garden scraps, coffee grounds, and egg shells layered with newspaper or straw. Then fill the top 4-6 inches with potting soil. The paper and straw will breakdown over time and help fill the box with good growing medium without the extra cost of potting soil. These boxes are great for mints, or herbs, anything you don't want spreading in the garden. Also, works great for lettuce, spinach, radishes and other small root crops.
Good luck
Just an FYI to give you an idea - I have a few small planter boxes around my house. I keep peppermint, chives, and thyme in mine. I also have one I fill with annuals.