On the ply for the front and back, the two 400mm x 1000mm lengths, measure in 150mm from each side and 70mm up and mark out your cutout on both sides then connect them with a straight line. For the sides pieces, the two 400 x 765mm lengths, mark in 134mm and 70mm up and mark up and connect with a straight line. You can make round edges at the corners by tracing around something circular like a spray paint can.