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.
Ok - so I've fit it pretty well I have to say. On top, its going from the very front, under the cpu/mobo, then all the way to the end. The bottom is similarly covered to the mod, without the bulge obviously.
Temps are
64° / 60°, so unfortunately it doesn't seem to have helped.
I live in a very hot country and during the day there's no A/C so room temp will be pretty warm, not sure if I'm comfortable leaving it running if I'm getting these temps in an AC'd room

Maybe I could manage it remotely and underclock if it gets too hot?
Bear in mind that the mod REDUCES the airflow over the sensors, so temp readings are not that accurate (even without the mod).
What only you can tell for your ambient is how well the machine is running .... e.g are you getting more HW errors with the mod, are you getting higher diffs / faster diff setting by the pool? If you are getting less errors (assuming you've OC'ed), then that is a good sign the mod is doing well.