The key design feature of this trolley (or dolly) is that the bottom box is snugly and thus firmly held in by friction. This means that it is very easy to use the bottom box to push or pull the trolley, and thus any boxes stacked on the first.
Process (all steps shown here as the process is dead simple):
1/ Find a bit of wood for the base, and cut it to the width of the box on it's longest side, plus the thickness of the two end pieces of wood.
2/ For the end pieces, the width should be slightly shorter than the actual width of the shortest side of the box. This means that the side pieces when attached will grip the box and stop it from moving.
3/ Then screw then end pieces to the base.
4/ Then measure the side pieces, cut and screw to ends of the end pieces.
5/ Then find four casters and screw to the four corners of the base board.
6/ Finally, push the box into the snug fitting wooden battens, and you're done.