Strengthen wireless signal with Al foil

If your router is somewhat distant from your computer and you would like a stronger signal, it may be quite easy to do.
Here's a video that discusses this topic.
However, I fail to see why the extender needs to be parabolic with the antenna at the focus. Certainly, if we were dealing with a receiver then a parabolic reflector with the receiver at the parabolic focus would be essential.
But in this case, all we want to do is to stop a portion of your wifi signals that may be going from your router, through the walls and out of your house, or into a part of your house where you don't need wifi.
Instead we'd prefer that these lost signals are directed to wherever you can use them.
What I did was ridiculously simple. Just an arc of foil placed part-way around the antenna on my router and set to reflect in the direction of my other computer. See the screenshot (yes, that is a large CRT monitor between the router and to where I'm trying to direct my radio signals. Certainly this is not advisable but it has never caused me any problems). Note that this took me less than 2 minutes to make.
So, how did it perform?
Well, I have a MacBook very close to the router (distance about 70 cms) so the signal strength is already quite high. As the signal level varies continuously, I took 15 readings (just type iwconfig in a terminal) with and a further 15 reading without my al-foil arc in place.

Average signal level WITHOUT arc = -13.1 dBm (range -10 to -17)
Average signal level WITH arc = -11.0 dBm (range -10 to -13)

Now I tried it on a Dell E520 desktop that was situated in a room alongside and at a distance of about 4 meters from the router. Here are the results I got with this (average of 35 readings with and 35 readings without the arc):

Average signal level WITHOUT arc = -58.8 dBm (range -51 to -62)
Average signal level WITH arc = -50.6 dBm (range -44 to -56)

Well, it's not bad for something that took two minutes to set up and cost less than €0.05 to make.