I' ve got a full size seat cushion on my rear rack. Cabela' s calls it a deluxe utility rear rack bag. It' s the kind that has the pockets on the backs and sides and they form a `U' facing the driver that helps hold the bow in it' s place. I lay my bow in front of the back pocket across the cushion with the quiver up. I then put two long bungy cords across the bow, side to side on the 4 wheeler. The bungy cords go between the stablilizer and the riser and also fits between the sights and the riser on my bow. You can use one bungy cord but I use two just in case one breaks. The cushion totally supports and cushions the bow from all vibrations.
I also carry my rifle the same way with the bungy cords going between the scope and the rifle. I' ve ridden my 4 wheeler in extremely rough conditions with my bow or rifle in that configuration and I' ve never hurt them or knocked the sights off. You do have to be carefull on a long bow or gun that you don' t swipe the side of the 4 wheelers with trees. The only thing that I carry that is a problem in that regards is my muzzleloader which has a 28 inch barrel.
I' ve not found anything better.