For a miter joint to be strong, it should have a key to provide more glue surface across the joint. Miters are end grain and end grain does not bond well due to lack of surface area. The key is bonded face grain to face grain and is very strong. To make this joint I use a 1/4 in dado setup on the table saw with the blade all of the way up. In order to make the cut I made jigs to hold the hatch.
There are three jigs for the three corner angles. They are simply 3/4 in particle board that I pinned plywood strips to at the appropriate angle. The hatch can now be passed over the blade and the key slot is cut,
To make the key I usually resaw a piece of scrap that is the right width, making two halves, then plane these to 1/4 in. When I get close to the finished thickness, I just check to see if the key fits in the slot easily and keep making planer passes till it fits nicely. A block plane is also helpful for fitting the keys.
Use epoxy to glue in the keys and after it sets up I trim the excess on the band saw and use a block plane to bring it flush.