Get a quilt. Lots of discussion about down or synthetics on backpackinglight.com. You should be able to stay well under 2 pounds with a 30 degree product. Down will pack to grapefruit size, poly fill will be more football sized.
I have used a kelty synthetic 30 degree bag for kayak self support and just got a Kelty drydown feather 30 degree bag for future trips.
Both pack down really small and do the job great for cool to cold river nights. I will augment either bag with a warm hat, fleece / capalene top, longs and wool sox when it gets down close to freezing. Have added my dried out fleece paddle top as well as needed. As Randaddy posted, the down bag packs down a bit smaller.
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