I thought for a while about doing it in stainless steel tubing before reverting to wood.   Vertical beams in the four corners with screws running into the fore and aft wood bulkheads.  Then horizontal beams running down each side screwed into those verticals.   The top horizontal is just below the roof. The lower on rests on the fixed fore locker but is also screwed to the verticals.  I then have one more set of horizontals just over half way up.   

At the rear I then have two sets of shelves on either side.  On starboard they run fore-aft and on the port side they run port-starboard.   This gives large storage for bikes and smaller storage for various stuff.  It also leaves room around the ladder and does not block the hatch access at all.   

Finally I have hooks on the forward port and starboard sides for spare ropes and other stuff that will hang.  This means I still have easy access to the fore lockers for anchor and bow thruster.

I am really pleased with the result.  It uses the large space for easy access storage whilst still retaining the roomy space for fenders and sails and dinghies etc. 

Pictures attached .....

Wow, Colin this gives me a lot of inputs. Where do you the fenders? We don't want to climb into the Lazarette to grab them.

Unfortunately I will only have time in February to see our 508 and work on her for the first as the dealer screwed up the exposition of our yacht at Dusseldorf. But the guys of Wendel and Rados will assist me.