There, ma friend, lies the root to the rise in temps ..... try using simple card-board over the top and bottom (while you sort out the 3D printing) and you will see a drop in temps.
Should the cardboard on top sit just above the heatsink and not extend across to the sides? Or should I try to sit it on top of the PCB board, extending to the side panels?
Thanks
Fit it just like the mod is, linearly aligned with the fan ducts aka on top of the heat-sinks. The idea of the mod is to get most air through the heatsinks rather than escaping through the sides and top.
I am assuming here that it is a temporary fix as card-board can be a fire hazard IF the the dreaded S5 overheat bug hits you.