Hold it is not accessible from the cabin (no matter what you might see in movies).
Cabin crew rest area also not really an option ? for most planes would be difficult to get a body in there in the first place and next to impossible to get it out once rigor mortis has set in.
Main consideration is how to remove the body from the plane after the flight (I have seen food trolley being used for this purpose ), that probably was why it was put in first class ? closer to the door and more room to maneuver.
Another problem is that corpses will often release ?bodily fluids? ? that is why some airlines will wrap the body in plastic and/or (as mentioned) put it in one of the toilet cubicles.
Really no good solution for this problem that I am aware of. If there were, airlines would have figured it out by now ? quite a few people die on planes.