This is what it should look like together roughly. Generally I put a piece of plywood (generally MDF, even though it isn't great when wet, this shouldn't be getting too wet, and can always be sealed with polyurethane) on the top, and on the bottom sitting on top of the 2x4s that are on the bottom running lengthwise. The bottom what is cut with a jig saw to get it to fit on there very snug.
Like 6 FEET LOL