I think you can re-use the K55's top panel as the top panel for the new tophat. I mean, the tophat will basically be a rectangular ring style device, with its bottom sitting on top of the K55 (with top panel removed) with screw key holes matching the existing screws. The top hat will use the existing top panel to attach the 240 fan-rad.
SGPC K99 is so similar to K55 but it is just taller to allow a 240 fan-rad at the top. You can watch this video (especially beginning around 5:00) and get inspired to mod the existing top panel to receive the 240 fan-rad. Basically, you just need to have some screw holes at the right position to screw on the fan-rad.
Or, if you have not bought the K55, maybe you just buy the K99 and skip the tophat part to call it a good day.