A floating lid still needs an island and should be sized the same whether using a one-piece heatsink or floating lid. The primary difference is in the sizing and construction of the base or heat spreader portion for the floating lid. While the base of the floating lid can be sized larger than the package body, it generally is sized to match the body. The stiffener ring also has guide holes at the four corners of the device that with appropriately placed pins on the floating lid, guides it for proper alignment to the device package without the need for additional attachments. The top surface is flat to allow interface to any additional thermal management component like an off-the-shelf heatsink. The following figure depicts an example floating lid mechanical drawing.
Figure 1. Example Floating Lid Diagram