Ok, so what are they supposed to be made of?

Lots of holy awesome Allah-ness...Duh...
Seriously, the nature of Allah is a huge part of salafi creed. They make a huge distinction between affirming the attributes that Allah is said to have in the Qur’an and anthropomorphisizing him.
They say that Allah has hands, as is mentioned in the Qur’an, and that we can understand the meaning of the word “hands” linguistically. Allah’s hands will be unlike anything in creation, as there is nothing like unto him, but they will still fit the definition of hands. It is haraam to ask how they work “takyeef” or to try to interpret them “ta’weel” or to try to make analogies for them “tamtheel.”
This is a central part of salafi dogma.