Your particular style of sleeping, with one straight and one bent leg would clearly make mummy bags a problem. I haven't used a quilt, but there are quite a lot of experienced people here who swear by them. I'd look into that option before I'd look at a rectangular bag. The drawback of a rectangle is that it creates a very large air space that your body must keep warm. A quilt should be able to wrap you more snugly and eliminate that inefficiency.