Strap it to the back of your pack vertically. I have actually had straps for just my bow sewn on. You can also add a pouch (bucket) so your bow sits in a holder for the bottom of the bow(Badlands). Make sure the bottom of the bow is about 10" higher than the bottom of your pack, so when you set it down it does not sit on the bow cams. DO NOT make the mistake of strapping anything on the string-it will rub through!!
Another option would be to make a bow case from silnylon or even Cuben with straps sewn on so you can custom attach it to your pack.