I've wanted one of these for quite some time but it's one of those things you never quite get around to doing. But three kids later I'm finally making one! It was very simple and if I hadn't run out of bias tape, would have completed it about 45 minutes after I started it. I seem to have reoccuring issues with not having enough bias tape. I think that would be partly due to trying to figure out what I need for notions for a project I haven't really started or even drafted out while I'm standing in the sewing department of walmart....
I've seen these here and there on different websites and most have rounded corners. I just cut two pieces of fabric (top is flannel and bottom is a quilted cotton) 40 inches long by the width of the fabric and bound them with bias tape. Then for the strapes, I laid the blanket down into the carseat and marked the straps and then basically made two long button holes for the shoulder straps and one large button hole for the strap that comes up between the legs.
I have more material to make a lighter weight one for summer and I'm still working on my new dresses, shirts and pants but I got sidetracked the other day while ironing material by my UFO (unfinished objects) bin and started to do some putzy work on finally getting some of that finished up. I even found a skirt where I cut all of the fabric pieces but don't know what pattern I cut it from so I'm not sure how it goes together. But I think that'll work because if I remember, I have another skirt like that but don't like the yoke so now I can maybe fix the waist line and get rid of the yoke.