For a "hunting" back quiver I use leather broadhead booties that are tied together. You can draw one of the arrows and the rest hold the booty in place in the bottom of your quiver. To really quiet them down you can tie all but one arrow to the top of the quiver.
This method was written up in an article in Instinctive Archer Magazine some years ago.
Otherwise you can fill the bottom of your quiver with things like dry rice, beans, oats or foam to keep the broadheads from banging into one another.
Also, a back quiver that is properly broken in will collapse in the middle across the curve of your back and hold the arrows pretty well just by friction.
Hope that helps.