Might take a look at Montbell. Their bag stretches, gives you a roomier feel.
To the O.P.:
Are you talking fully rectangular....as in no taper at all? For 15F you'll need a heck of a bag if it's rectangular. Might check Cabela's Alaskan Guide bags. Heavy, but warm and roomy. Not for backpacking.
Western Mountaineering Sequoia rated 5*. Badger is a large cut semi mummy at 15*
I've got a big Agnes lost ranger that is a lot more roomy, I can roll around a little. It's a 15* but I get cold when it gets down to 20*
My Lost Ranger is about a 40 degree bag. The zero degree Storm King might work at 15 degrees.