Gerufa u’tmuna can be accomplished by any covering of the fire and he’ker – in past tin was used because it was cheap and easy to use, but any material (for safety – fireproof and heat conducting) can be used.
BTW – A layer of aluminium foil would not be “kosher” for a blech because it would be bottel to the oven (unless several layers are used).
Seriously though, I believe the ones that are commonly sold in stores are galvanized steel (similar to the air conditioning ducts in houses, but thicker).
Based on thermal conductivity, melting point, and cost, it seems to be the most reasonable option.