I use old microwave ovens & self tapping screw the door shut.
They turn out to be about 15kgs, about the weight of a good car battery.

Thats about 33Lbs for people who like to brag about the bigger weights that they can lift.

For other bigger but not too big bits of lightgauge, I have been using washing machine cases, just finding something to fit it snugly for a good weight bearing bottom end & then slowly filling it up to the top.
It can still be shifted with a sack barrow, is cubic so it does not waste much space n the shed & fits a trailer nicely, and tips off it nicely at the scrapyard.