Here is the start, a 24" x 48" piece cut from the end of an 8' x 4' x 9mm sheet of exterior grade plywood.
Using the method described previously I clamp the guide to the work piece at the correct point and then make the required cut using the saw resulting in a straight cut for the first piece.
The second piece is cut from the other end of the plywood so that two of the pieces have a good straight and true edge, I use these for the sides of the box. Then the third piece is cut from the remaining plywood leaving a piece that is 24" long but less than 12" wide. This will form the bottom of the box and needs to be cut smaller so that it fits inside the finished box. Theoretically the amount to reduce the width and length by is twice the thickness of the plywood or about 3/4", but this will give an extraordinary tight fit so I reduce the bottom piece by a further 1/8" making the required piece 23 1/8" long by 11 1/8" wide. The corners are also cut off as per the original design.
Whenever I'm cutting across the grain on plywood I score two cuts in the veneer with a sharp knife about 2mm apart before cutting the plywood. The saw blade cuts the wood between the scoring and prevents the top veneer from splintering as you can see from the next set of photos. For a comparison, the last photo shows two pieces cut across the grain with and without the scoring.
You should now have three identically sized pieces of plywood for the two long sides and the top and the bottom piece.