Nice setup mate (must get myself a temp probe!)
Just a thought, do you still have the shelves from the fridge? If so, you might get a longer cool by putting the top shelf in (with insulation on top of it) to effectively bring the fridge ceiling down to just above your fermenter. The cold air has less fridge to chill and you might get to spend more time drinking beer and less time swopping frozen bottles
It worked for me when I reduced the height of my homemade 'fridge' and I can now keep an even 14 degrees (according to the stick-on temp strip on the fermy) with one 2litre frozen water bottle replenished each day.
Hope every brew's a winner