Slow cooker? Sous vide is out for me due to plastic....don't trust it.
I suppose you could get yourself one of those insulated thermal pots I've seen on Amazon. You heat it to temperature, put it in the thermal casing, then leave it for hours. Of course, if you don't heat it to boiling at the start, It might cool rather than finish cooking in the normal time frame. You could probably remove it partway through and heat it some more.