There are at least three ways to cut this section. You could do a straight 90 deg. cut, or you could go for the more traditional curved cut. Remember, if you are making the box from a single piece of timber, you must cut the curve or shoulder before putting on the front skin.
I chose to go for a shoulder cut with a 45 deg. angle. The shoulder is 18mm wide to accommodate the hinge batten that will be glued and pinned to the plywood edges crated by cutting the two pieces.
I carefully, very carefully, marked four pin positions and used my square to make sure the drill was perpendicular in both planes as I drilled the holes.
The fit was very good, so I glued and clamped the assembly together.
After that I added two battens inside the lid and carcass to take the hinges. Leaving this all to dry, I went put to buy the hinges and catches and handle.
I chose a lie flat case handle that I could bolt to the top of the case and some simple case catches.
I took my time setting out and cutting the hinge rebates. I've always found hinges a challenge, getting the rebate the right depth and then making the fine adjustments that need to be made just takes patience. I can see why someone invented the cabinet hinge that kitchen manufacturers are so keen to use.
Before I packed in for the day, I applied woodfiller where needed ready for sanding and finishing. The next job is to make the trays or till to go in the top of the box and decide what to keep in it and how to fix them in position.