It is a decent low tech idea. It would be pretty awesome to have a sleeping nook. Even if you bring some small amount of electric heat or ventilation into the system, like repurposing an old bathroom vent fan, you could perfectly air condition just that one 8x6x5 box and let the rest of the house drop into the low 50s at night and be both comfortable and save money, then use the warmth of the day to warm the house back up for use.
Might be a fun project.