Combine first 7 ingredients in a large saucepan; bring to a boil. Reduce heat; simmer 10 minutes, stirring occasionally. Set aside.
Cut cake into 1-inch-thick slices; cut each slice in half diagonally to make 2 triangles. Line the bottom and sides of a 3-quart straight-sided glass bowl or trifle bowl with half of the cake pieces; spoon half of peach mixture into cake-lined bowl. Arrange half of remaining cake pieces over the peach mixture, and spoon remaining peach mixture over cake. Top with remaining cake pieces.
Place plastic wrap on surface of cake pieces, and place a small plate on top of plastic wrap; place a 1-pound can on plate to firmly press mixture. Chill for 8 hours. Remove plate and plastic wrap. Garnish with additional fresh berries, if desired.