Visit your local steel stockist, and beg some offcuts of steel hollow box section with a wall thickness the thickness you want. Get it home and chuck each piece onto your open (or closed) fire, bar b que or bonfire. Get them up to red heat for a while and let them cool slooowwly. This should stress relieve them.
See the forum tip “Cutting steel plate” and cut the box section along its length giving you some nice bits of metal. Why cook them – if you don’t, they curl up like a banana as the rolled stresses are relieved.