House all of the actual components within the center of the sandwich. The sandwich can be constructed out of any material such as acrylic, or any other plastic material so that the food items match the clean and simple aesthetic of the sandvich. Thus, there will be in effect a box area inside the mock sandvich which will house all the components as they would normal be installed. Thus, instead of creating the whole sandwich, the parts of the sandwich would not be whole. This means that instead of having a full tomato as a circle, you'd only have a half-circle or whatever other shape the space would allow for with a core center area being preserved in the form of a standard metal box acting as a mounting plate and a mini case within the outside "sandvich" casing.
It is however possible to merely stack the food pieces on top of each other creating a box without the need for a metal box and only a small custom motherboard tray to mount the board on. This would allow the case to be created using transparent acrylic so that the components could be clearly displayed while still maintaining the aesthetic and shape of a sandvich.
The optical drive's disk tray could be hidden as a piece of cheese, lettuce, ham or tomato etc. so that the food item would slide out when a button is pressed which could also be hidden inside as another food item.
The olive's red pit could act as a power button with a small thin dowel rod acting as the toothpick could conceal the required wiring within.
It's important to realize that the completed build is not meant to look like a lifelike, realistic sandwich, but is meant to embody the essence of Team Fortress 2 and the Sandvich itself, by being a caricature of a real sandwich. Thus the case utilizes clean cut acrylic pieces instead of custom paint jobs or stealthed items which mimic the texture of the actual food etc.