I wasn't keen on the hairband method either but if you experiment while LO is in it you should be able to get it to work to try out. Just put a hairband round the body of the carrier (so you need to thread the straps through it, then slide it up the body of the carrier a bit) and move it to where you think it needs to be narrower, where LO's legs will stick out. A ribbon may be easier to adjust cos you can tighten or loosen it as needed. When LO is in the carrier with legs out, just move the hairband/ribbon up or down the carrier to the right place.
I did that at first to see if it would help and if LO would be comfortable with legs out, but then I sewed the carrier a bit narrower as a more permanent solution. I did this by folding the waistband with a couple of pleats and stitching them down securely. I did it by hand with stitches that are visible on the inside so they're easy to remove later on. Where the carrier switches from the waistband to the front panel, it sorts itself out. I did this with my first LO and took them out when she was big enough.