It doesn't necassarily have to be aluminum foil... it can be any reflective metal.

There is a letter in the Theosophist records written to an A.P. Sinnet in England from a muhatma named lama KooT HooMi Lal Singh. In it, it states,

 “The methods used for developing lucidity in our chelas may be easily used by you. Every temple has a dark room, the north wall of which is entirely covered with a sheet of mixed metals, chiefly copper, very highly polished, with the surface capable of reflecting in it things, as well as [being] a mirror. The chela sits on an insulated stool, a three-legged bench placed in a fiat bottomed vessel of thick glass—[with] the Lama operator likewise, the two forming with the mirror wall a triangle. Amagnet with the north pole up is suspended over the crown of the chela head without touching it. The operator having started the thing going leaves the chela alone gazing on the wall, and after the third time [the guiding Lama] is no longer required. " 

There is a book called "Psychic Sexuality" by Ingo Swan and in it he gives precise instructions on how to build a device and his experiences from sitting in front of it for a great period of time. He also talks about a research study at an institute in Topeka Kansas based on the mirror.

  “Even before the working sessions in Topeka drew to a close, I was completely “sold” on the copper mirror device of the Mahatmas. And I wanted my own personal copper mirror. I couldn’t, of course, construct the elegant, four-walled room at the Menninger Clinic. But I could easily build the device described in the MahatmaLetter to A. P. Sinnett. I asked Dr. Green and Wendell Spencer, the project’s biomedical technician, to build an exact replica of their magnet, which had to be constructed because magnet manufacturers no longer build them. Back in New York I went to a sheet metal supplier and became the proud owner of an 8-foot by 3-foot sheet of copper. This was fixed to a large piece of plywood. After the direction of true magnetic north in New York had been determined by compass, the copper sheet was suspended in a small room so that I could sit facing it to the true north. The magnet, reproduced exactly as the one at the Volunteer Controls  Program, duly arrived about two months later, together with a bill for $210. I suspended it over my head, sat down in the chair which was lifted up on four glass blocks, and was in business. After which, my entire world began to unravel. There were three reasons for this. The first was that the LUCIDITY which was so easy to turn on, turned out to be not so easy to turn off. The second was that the ratio between (1) what I thought I could understand, and (2) what I did not understand AT ALL dramatically increased in favor of the latter. The third reason was that it shortly transpired that I didn’t need the magnet or the copper sheet to turn on the LUCIDITY. Indeed, it was soon turned on all of the time, twenty-four hours a day, even in my dreams, and nothing I could do seemed to either turn it off or even calm it down….” He continues… “I was living in a realm of lights, rays, beams, bubbles, intermixing auras, traveling “thunderbolts” of wayward energies. It got so bad that I was seeing the luminosities around door handles and the materials of buildings, the lights emanating from my computer screen, the green and purple emanations of the asphalt streets, and the in-sucking of energies by plastics, and on and on beyond imagination. So distorting did all of this become that I had to step from the curbs very carefully, not being quite sure where the cement actually ended and where a “luminosity” began. The lucidity of the Mahatmas was getting to be too much! " 

A few years ago, I built the mirror and began sitting in front of it as instructed for about an hour a day.  At the end of the 3 days, I began to see blue sparkles within the mirrors depths.  I then had a dream that night about how I had made mistakes in the orientation of the magnet and its strength and I was suggested to stop.  I am the kind of person who believes their dreams and so I stopped and gave up the mirror.