I'd steer clear away from aluminum... from what I know, it can be toxic
to fish... for that matter, I'd stay away from anything metal (except
stainless steel)... If they're smaller fish.... try turning some black
plastic milk crates upside down in the pond (black camouflages better
than other colors), bore large holes in the sides if need-be.... weigh
them down with bricks, or even better... set some pots of aquatic
plants or lotuses/lilies upon them to keep them down. Aside from
that, you could buy a large black plastic stock tub from a
hardware/feed store and manually cut holes in the sides to accomodate
your fishes' comings and goings (and hidings).