A bit of a morbid tip of the week but one that could come in handy and that is not quite obvious to find.
While we have several "coffin" or "casket" props, these are not open or have characters inside. However, we do have the option to do this under a different menu and there's a bit of irony in creating a deceased character.
First, you can add your chosen character to your current scene. You can then click on your character to open up the character settings menu and select the "Actions" tab (yes, dying is an action but wait, there's more).
Under the "Daily Activities" section (the real irony), you can type "dead in coffin" and click on the one available action so that it can be applied to your chosen character. Please note this action is not available for children characters.