I have built this 10" x 8" x 2" sound box to house the Eu-Long cookie tin outer lid. I tried to cut round corners, but gave up. On this build at least, the corners will be mini sound holes. At first I was going to have the square resonator lid sit on the 6 "flanges" shown in this photo. They were made by gluing together two lengths of trim wood with one piece being wide than the other to provide a "ledge" to the cookie tin to rest on (it will be held in by string tension through the bridge). There was a slight gap, so I filled it with a long strip of 2mm plywood (the kind I usually use for my sound boards). I did this to both the right and left sides of the sound boxes and these 2 "shims held the lid quite tightly in the sound box. Later, I found I could make the sound board out of 4 pieces of 2mm plywood and could wedge them under the rim of the outer cookie tin, so that's what I did.