The North Face Goliath is a wide bag, but I don't know that it comes in anything other than a zero degree rating. Kifaru makes a variety of bags with width and length options although I've never actually seen one. It's one of my wish-for items.

But if there is no backpacking and size and overheating are issues, I would suggest going to a department store and getting two inexpensive rectangular sleeping bags. This will allow either one or two bags to be used depending on temperature, will be wider, and probably even less expensive. One can be zipped up and the other used as a quilt over top that is easily removed if it gets too hot. The only real downside might be bulk and weight.