I am not sure that 'bending' the metal is entirely necessary as in terms of focussing. A plane sheet of metal will act as a reflector giving a signal distribution that is the same as if you had two antenna, the real one plus its 'image' in the metal (as if metal was a mirror). If you put the metal too close to the antenna (near field) you will screw up the impedances and this would be a bad thing. Think you would want it a significant fraction of a wavelength away, which at 2.4 Ghz with c= 3x10^8 is.....a bit bigger than 0.1m so say a fifth of that, 20cm. Now the issue is how much of the radiated energy going 'backward' towards the metal is actually intercepted,so want a big sheet.
There is a 180 degree phase shift from the image (to null out the field at the metal), so side on will give no signal. So if you are trying for constructive interference don't forget to add a lamda over 2 to allow for this.
EDIt: In terms of health, can't say I would be too worried at the signal strengths involved. The most sensitive part of your anatomy (above trouser level) are your eyes. High radiation causes the humours in the eyes to denature and they go opaque as per cooked egg whites. So desist at the first signs of blindness would be my advice. Other thing is that I suspect half the wifi issues I experience are more to do with multipath interference than raw signal strength, though such interference will get worse at low signal levels. So, check out any big metal things (eg range cooker
) in LOS between router and computer.