Here is the first working build of the barrel holder which underwent its first sea trial on Sunday (8th Feb 2015). The basic concept is a barrel which sits on a hinged arm. The hinge location is over the rear of the daggerboard slot and there is a 'plug' which sits in the daggerboard slot and benefits from the rigidity of the board in this area thereby providing support against lateral forces (in other words it helps prevent the barrel falling to one side or the other). Whilst sailing the hinged arm fits snugly into a 'saddle' held in place by the fin bolts. A nice feature is that the arm is hinged so that it can come upright and to the centre of the board, making on-the-water barrel access possible.
There is room for your back foot toes under the arm whilst using the reaching straps.
The sea trials were a success and hinderance to sailing was minor: on a tight reach the sail couldn't quite tuck in as tight as you would like, and on the exit to the carve gybe the arm is where your foot wants to go. I can live with that as the main points of sailing were absolutely fine.
I loaded the barrel (the test barrel being a 25 litre paint container) up with 6 kilos. You notice it. Upwind the nose is a little higher and getting planing on a reach maybe requires a couple of knots more wind. I'm going to adjust the design to bring the weight a little bit further forward and refine the barrel for sea trial 2.