Lots of things to consider here. First, if you handle is thick enough you can use a round file to cut a groove to help hold the arrow in place and for arrow clearance. Then just something to hold the arrow from below, as Ralph suggests. You can easily carve a little shelf and glue it on below the handle notch, or drill a small shallow hole not too deep into the handle and glue in a peg for an arrow shelf. Thing about just a shelf with no handle groove for the arrow is that you’re pretty much required to cant the bow to keep the arrow from falling off. Even when using your finger for a shelf as per traditional longbow shooting style, there’s enough friction there that you can shoot with the bow vertical and not lose the arrow. Not so with a shelf. I never shoot better with a Hill style longbow than when shooting off a finger, or “the knuckles” as we say. What does help is to have some sort of a reference to help you get your hand in exactly the same location every time.