I'm lazy enough to use a single loop and bowyers knot all the time.
For a long tiller string I use para cord with a slip knot in each end. For a short tiller string I use a B-50 string with one loop and bowyers knot. This string is long enough(ans strong enough) to use while making any length bow. When I hit brace height I make a string for that bow with a single loop and a bowyers knot.
When I first make the permanent string I prestretch it then tie off the bowyers knot , put it on the bow and stretch it again by placing the bow across my knees and push down and out on the bow's tips. After I'm sure the string has stretched enough I tie an overhand knot in the tag end up against the bowyers knot and let the rest frey. Once I get a string(knot and string) broke in I rarely have trouble with it slipping. If it does stretch a bit I unhook the bowyers knot from the tip and give it a few twist until it gets to my fistmele.