You could try a scotty base mount . Cut the mount length wise in half then drill and tap a thread for a locking bolt and use a piece of stainless as a cross bar.If this sits too low on the deck a packing plate can be used to increase height. The packing plates are made from plastic and are used to level prefab concrete walls in buildings and quite often for levelling pallet racking, and are cheap to buy. These can be found at shelving and storage companies or if you have a Ramset supplier close by.Come in a range of colours ( each colour relates to thickness...EG a 10 mm plate from memory is black 5mm plate is yellow. size ranges from 1mm to 25mm.