Combined improvements Corner-front and Corner-Back

.4 nozzle. .2 layer height. Default print settings. There were a couple remixes of the top that fixed problems so I combined them and added my own fix. @zktec_3322325 fixed the entire top for better stacking but changed the top to a single piece and changed the screw types. I kept the top fix but reverted to the original M3 screws and two piece corner-front and corner-back. @JayTea_3263329 noticed that the original model has a transition curve from the inside vertical to bottom that interferes with a good fit with the cereal box. I removed that transition but kept the original front thickness rather than thin the entire front screw area like @Jaytea did in his remix. I noticed that the front of the cereal box had 5 mm of clearance on each side that made the corner front part wiggle around while trying to drill and screw. I added tapered rails to the inside edges to keep the Corner Front part secure.