Create a former the full width (or is that depth/thickness) of the side frame - green colour.
Cut narrow strips of thin (?copy) paper and laminate them in several layers around that former -blue and black colour.
(without sticking the laminates to the former !!)
Round edges.
Replace former with side wall and details.
Have fun.
Don't blame me if it don't work.
Further thought, you might want to make up some external formers as well,