Now you have two halves of the box, they need to be joined together with the piano hinge. Cut a length of hinge with screw holes evenly spaced at each end and fold the hinge flat against the box joint between the two 10cm panels and use short 1cm screws to fix. Marking the screw holes with a sharp point beforehand will make putting the screws in easier.
At the other side of the box, the clasp has to be fitted. Fit the hook on the short 2cm lid section first, using short 1cm screws, and then with the loop of the clasp over hook, pull the top and bottom of box together and mark where the centre of the screw holes lies. Release the loop and put a hole about 1mm under this position to put the screw in and fix the clasp there. This ensures that when the clasp is closed it will have a small amount of tension to hold the lid down.
On the base, measure an equal distance in from the four corners so that the rubber feet you have chosen just touch the edges of the box and screw the feet on.
On the lid fit the strap handle across the centre of the box lid from side to side, centring it up across the length.so that it will carry easily.
Fit four more rubber feet to the lid, measuring an extra 'foot' width in from the front and rear so that when two boxes are stacked, the feet overlap. These extra feet will ensure that the lid sits evenly when open.
I've already fitted an IEC socket/fuse/switch to one of the boxes for its internal power supply and will be adding more in part 2.