Just remember that when you start modifying something like this you may end up with a boat that becomes slightly less desireable than one that was left stock.
If you add a refrigeration unit to the existing box, you may not need additional storage space. The evaporator plate or holding plates are very small and can keep you box cold without the need for ice (they can even make ice cubes.
Unfortunately I do not remember this model, so I may be off the mark. If you decide to go to refrigeration, I suggest that you take a look at an Isotherm SP unit.