My first thought, after seeing your pictures is, it's going to be awfully heavy once you get it filled with soil, plants and start watering. I'd recommend you put in some cross-bracing to prevent the box from sagging outwards in the middle. You might also need more support underneath. Be sure you also have plenty of drainage holes - some out the sides as well as the bottom might be a good idea.
I have a 3ft. x 12ft. raised bed made of Trex that sits on the ground. When my friend built it for me, he put in X supports across at two points inside the box that are now covered with soil. They hold the sides of the box so it has not sagged outwards in 9 years of use.
Also, are you sure that composite board will hold up to the constant moisture of a vegetable bed? It might have been wiser to build it with marine plywood that is meant to be wet.
Last thought for right now, is the sheep manure thoroughly composted? It can be very high in nitrogen and salts, which can burn young transplants. Be sure you mix all your soil components thoroughly so that the manure doesn't come into too much direct contact with your young plants.
