The shelf material I use usually does a good job of keeping the arrow on.If not,I'll leave a slight space between the shelf and the window(riser) when putting on both the shelf and side plate,this usually allows the arrow to sit in a slight dip or cradle.You can also put a thin piece of material under your shelf material towards the outside of the shelf to slightly raise it to keep the arrow on.
If all else fails,I screw my bow holder in a little crooked,pointed downward slightly,and it helps to hold the arrow on........