The slats are 1 x 3 red oak which are held together using 1/4 x 2 underneath. I did not attach them to the frame but instead measured and placed the 1/4 x 2's very tight against the inside frame that runs parallel with them which gave an extremely snug fit. Even though they are just held in with friction I was able to get a snug enough fit that they do not easily come out. They actually need to be pushed down with a little force to get them to click in place.
The bottom is two separate pieces because of the horizontal support bar. One set of two slats in the back and one set in the front.
Here is a picture with the yellow representing where the 1/4 x 2's are placed on the bottom of each shelf.
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