Mezuzah on an aluminum door

Hello Oren, here is a response from the ‘Hovat Hadar’ (On the Mezuzah) by R’ Yaakov Blau z”l (member of the Badz Eda Haredit), p’ 77. It seems that he permits the mezuzah to be placed outside the door space, but is satisfied with the blessing, since this is subject to dispute. He also adds more details:

”If he cannot place the mezuzah in the door space, he can place it outside the space, either inside or outside near the door space, a tap. The removal of a tap is invalid…. And in the comments, he noted the source of the ruling: ” ‘Little Hand’, and so on in the book of Shech (Rev. 3:11) in the name of the delicacies of the king. Who is in the clothing that wrote… invalid. And regarding the blessing, since Shech simply did so, it is possible that he could recite the blessing. But in the soul of a man he wrote … at the Wall that is not in the space of the door, he did nothing, according to the scripture.

And on pages 3 and 4 in the notes he concluded that it is better to set the mezuzah in such a position – from the outside rather than the inside. And when it is set from the outside, its head should be set near the door and its end should be set diagonally away, and if this is not possible and it is set from the inside, he should do the opposite, its end should be near the door and its head should be set diagonally away.