I have summarised below what I have done with my aft hatch cover. In brief I could not find a device on the market which was suitable for both security of contents and ensuring that the aft hatch cover did not come off.
I therefore bolted 2 small D-rings with backing boards and washers to the deck on either side of the hatch. I purchased a steel bar which was drilled at either end so that it can be connected to the D-rings with either a padlock (for security on a drying mooring) or quick-release shackles for use at sea. I drilled 3 holes through the bar and the hatch lid which were also reinforced by backing plates and Sikaflexed to avoid water ingress. This arrangement gives security and by opening only one end of the bar at sea there is, hopefully no danger of losing the hatch lid overboard. It also appears to apply an element of pressure on the lid, thereby helping to ensure it is well sealed around the edges.
I also drilled out some of the metal plates under the hatch clips and replaced with thicker plates in order to apply greater pressure around the edges.
I was initially concerned that this arrangement might cause problems with the main sheet, which is aft-mounted, but I have since done 5 day sails and it does not appear to be a problem, even after attaching a boom strap 18 inches in from the end of the boom to avoid the main sheet catching on the engine, when mounted, or the corners of the transom.
Overall I am happy with the end result. A bit Heath Robinson’ish you might think but it doesn’t look too bad – and it appears to achieve my multiple objectives.