We like people who go into confessional mode here. Anything else you care to share? But what you say makes perfect sense. Im a knees-up side-sleeper, and that doesnt always work well in mummies, either.
So youre looking for about a 20-degree bag? That should get you through the shoulder seasons in the Rockies. One possibility is the Big Agnes Roxy Ann +15 ($200; bigagnes.com). Its a womens-specific down bag rated to 15 degrees. It has quite a roomy cut, so you should be able to do your can-opener thing in comfort. The one down side is that its fairly heavy, at nearly three pounds.
The best design for sleep comfort is semi-rectangular: wider than a mummy, but not a full rectangle. Alas, thats a part of the market that doesnt get much attention, and most bags in this category tend to be heavy and made with inexpensive synthetic fill. Nothing wrong with them, theyre just best for car-camping.
But there is one exception. In my humble opinion, the very best light/warm/roomy sleeping bag is the Western Mountaineering Sycamore MF (westernmountaineering.com). Its rated to 25 degrees, has loads of 850-fill down, and is made with a roomy semi-rectangular design. Weight is two pounds, and it packs down beautifully. I should think it would get you through a few chilly spring or fall nights. The price is $345, but construction and materials are impeccable and it will last you many, many years.
The 2008 Winter Outside Buyers Guide is now online. From snow sports to trail-running to camping, get reviews of more than 300 new gear must-haves.