To finish off the mug holder I fitted two pieces of wood to the outboard edge of the holder. These act as stand-offs and prevent the holder from moving around on the shelf.
The completed holder.
With stand-offs at both ends...
So why did I not just screw the holder to the shelf?
Had I not lived on board a sailing boat for some years this is probably what I would have done, but experience has shown me that this would have been the wrong thing to do. The primary reason for not fixing the holder is that if you do so and something falls in through the holes it is somewhat difficult to get it out again especially if it rolls into one of the corners unde the holder. This way I can easily lift out the holder, retrieve the offending item and replace the rack, no fuss, no bother.
The secondary reason is that right now I don't know where most things are going to go. I'm putting the mugs, plates and bowls where I have space for them close to the galley but in a season or two of using the boat I may well decided that this is not the best place for them and want to put them somewhere else. So, not permanently fixing things down at this point is an advantage.