Using the old Coleman cooler that came with the boat (doesn't hold ice well), a hold down system that was a little more robust than bungee (though I love bungee) was needed.  After some sketching, a hardwood retainer was decided upon with half circles on the two ends that would lock onto the stern railing.  My first thoughts were to use two strips to secure the locker and hence why I designed the rotating lock.  To make this lock, the filler piece would be cut on the center line to the railing and the outer (aft) piece set up to rotate on a single screw allowing easy locking and unlocking.  To keep the locking piece from rotating except when wanted to secure or unsecure the cooler,  a bungee was used (told you I loved bungee). 

The filler piece is made of oak and secured through the cooler back with four long screws thru 1/8 inch aluminum backer plates. 

Inside the cooler, there is just enough space for a three gal Honda fuel tank, a two gallon spare fuel can from Lowes Home Center and a Coleman fuel cannister as the locker will also be used for the propane locker.  Venting is done by using PVC pipe fittings.

The height is 28 inches, so two inches less than a bar stool. 

As I said, the first sketches provided for two retainer pieces but the swim ladder lock eliminates one above it which meant that the upper one would be no higher than the middle of the ice chest so I decided to try one.  The cooler locked well in position but was free to move up some and then if it rotated, it would be free.  This discovery changed my plans and the swivel lock was abandoned in the stead of simply twisting the ice chest to mount.  All that was left, was to hold it all down with a bungee.  I did so in a way that the bungee was integral to the oak retainer as I didn't want to hunt a bungee when securing the locker or deep six one.

My cooler had to be shifted a little to port to clear the motor when raised.  Using the hardwood retainer, it can be positioned off center to the stern rails without problem. 

Note: the screws seen in the edge aren't necessary to the final design (I made the original swivel piece fixed).  It should also be noted that the cooler lid hinges forward, necessary to avoid 2.5 inches of forward set to the cooler for the lid to clear the rails when opening if hinges are aft.

Construction is from an oak 1x 2.75 so a piece of oak flooring works fine.  Start with a piece a few inches longer than the distance between the stern rails so that the 1 inch holes can be drilled using a spade bit.  Take the length measurements carefully for the hole centers and remember to allow enough clearance for the swim ladder lock to rotate. Drill and then cut the length so that only one half of each hole remains. It may be necessary to saw a few degrees angle on the edge that fits to the ice chest to deal with its sloped sides.  Drill holes for bungee to route and for pegs to anchor the bungee.  Fit to the ice chest and secure with screws from the inside.  I used two 1/8" aluminum backer plates each holding two screws as the inside skin is quite thin.