Are those measurements along the back edges or the front edges. You didn't say what the cabinets are to be finished in...like will they be a wood look finish, painted, or a laminate? Other details need to be discussed such as what is at the far ends...walls, appliances, windows, or passage doors.
Generally speaking the floor and roof can be cut as one "L" shape, and then framed. The pieces can be cut from the same sheet, like puzzle pieces. If this cabinet arrangement is to fit two walls, being one piece may not fit perfectly if in one piece. As one open cabinet, you still might have doors to figure, and how to hang them. The same goes for shelves.
If you meant...no dividers, then you'll likely have to figure a face frame. Doors can be hung off of that, and the shelves can be dadoed into the ends, and to the back of the FF's. Or the shelves could be made adjustable. You said an "open" cabinet, so that should be cleared up to specify if they even have doors. So, there's a few variables to work out with details.
You could make them in two sections, which would allow the bottoms and tops to have the grain oriented in opposing directions. The floors and tops can be mitered, and fitted with splines. Their connection can be done with mated face frame members in the front, and standing members in the back corner to screw the two cabinets together. Both cabinets should have a rabbeted back installed, and a hangrail. I would prefer not to use a french cleat mount.