If we're talking thinish sheet and you already have the adjustable angle magnetic things you might find they still work 'through' the ally to a piece of steel on the other side. If the magnets aren't strong enough then use magnets on both sides.
Personally i hate magnetic clamps so tend to use a home made thing kinda like this...
or often just prop the thing up at about the right angle get a couple of tacks on it and then tweak. A selection of bits of angle and a pile of clamps is very versatile too
Nah, the pic i posted is nicked off the internet as i didn't have of my own to hand. My homemade version uses two bits of angle ally instead of plastic (as Shox i'd melt plastic ones designed for woodwork sooner or later)You made that yourself ??