Those flats look great for the price, but I build my own because I am a woodworker and I can repair them, unlike a plastic flat that once cracked, you tie together with wire or throw away. I use 1x4s and 1x6s, untreated pine. If you have the money cedar will last longer but what our shop offers is thinner so pine works. I have a weather proofer I use on other stuff but have skipped it so far. I can pick my size, the miter saw makes short work of the cutting. I have moved up to galvanized screws and Gorilla glue (regular wood glue dissolves in weather). I do the base as a combination of boards including 1x2 until there are narrow gaps. I line the entire bottom with window screen. I use feet on each corner that are made of cedar so even if they sit on the ground all year, they aren't just going to rot there. I'll post pics if I can find one in my collection, don't have an empty one sitting around. Find a basic picture, know some basic woodworking and it is yours. By number three you will have figured out a few design advancements. This will be my third year on my first box, doesn't look like it's going to bust at the seams this year.
Avoid MDF and plywood at all costs. Plywood may last a year or two or be gone by the end of this season. MDF was never meant to see so much as a wet rag. I know what it does in soak tests, still would never even try to use it outdoors with full sealing.