Convert the box to an editable poly (right-click > Convert To > Convert To Editable Poly) or add an Edit Poly modifier. Go to Vertex mode (shortcut: "1") and - still in the Top View - start moving the vertices so your box matches the guitar body.
Move the vertices around the rosette (the hollow body's hole) so you get a square poly with edges that are tangent to the hole circle. Go to Poly selection mode (shortcut: "4"), then select and delete the "hole" poly.
Select the edges that define the upper and the lower contour of the hollow body. Chamfer them 0.01 units. Then chamfer the resulting edges once more with an amount of 0.005.