At 30 degrees, I don’t expect that your rectangular bag will cut it – especially at Mt. Lemmon in the winter (which is in Arizona, right?) If you plan to do some backpacking at those temperatures (or lower), you definitely need a mummy bag so that you can trap heat against your body – which is actually what keeps you warm. Heat escapes out of the top of a rectangular bag – so they are much less effective at keeping you cozy.
If money is tight – get a synthetic mummy sleeping bag. They are heavier than down bags, but less expensive. You can check out http://backpackingsleepingbagsblog.com for some ideas. A low-end mummy bag sells for $100 or less, but shoot for 2-3 lbs if you can afford it – especially if you plan to backpack a lot.
Liners are available for less than $50 – and you could put one in your rectangular bed and get 10+ more degrees. Sleeping on a mat/pad will also help keep you a bit warmer (and add some comfort), but sooner or later you are going to have to bite the bullet and upgrade if you plan to do four-season backpacking. Maybe Santa can help in a few weeks. Regardless, enjoy the trip.
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