There is some play in the rudders's pivoting system so that at anchor it wobbles back and forth hitting the sides of the slot.

I made some wedges by buying a swim step tread made from that white Starboard stuff and cutting it in half lengthwise (one half for each rudder). There were already screw holes at each end that were just screaming the word "laynard". I used a belt sander and tapered them from one end to the other, down to about 1/16 inch. Here is a wedge sitting on its side on top of the swim step that it is normally stowed under.

Here is one of the wedges in use. I center the wheel/rudders and then just shove the wedge between the slot and the rudder swivel thingy (what is that anyway?). After watching things in here for far too long while at the dock (got quite a few comments), it appears that it is not the rudder itself that is banging, but instead it is the two-piece black aluminum "carrier" of the rudder. The part that swivels up to let the rudders kick up. Its a good thing this is, because there is no way to put the wedge down past this "carrier" piece and get to the rudder through this slot.

When not in use, the wedge can just sit in the locker, out of the way of any steering mechanism. It was suggested that I make a fixed holder for it so that it cannot jump around and get caught up in anything in a seaway. That is now on my "to do" list.