My shelf dimension is 36"W x 18"D.
I played around a lot in determining how deep to make the shelf, this is purely personal. I wanted it deep enough for a 14"x18" utility tray to sit on it, not hit the dome handle, and not feel too deep when belly' d up it it and flipping food. The 36" width still allows the cover to fit so I didn't need to trim that dimension. I chose the 14" size hinges so they would not be too close to the front edge of the shelf.
This left me enough scrap top material to make the filler block below the controller, and the front face for the lower shelf, (the rest of the lower shelf was made from the left-over scrap from the 2x8 board used for the mount).
BTW, I glued that filler block onto the mount. I shimmed it level with the top and put foam wedges between it and the pellet hopper so it hugged the folding top, until the glue dried. I used Clear Gorilla Glue.