mild steel is tough, but not strong.
rims are made from worked (rolled) steel, which gives it extra strength, and are not same alloy as mild steel..
could go with a higher carbon steel, like a medium high tensile (1030 or 1040), or a high tensile steel (4140)
or.. just go with whatever steel they have at your local steel shop in that diameter
25mm mild bar is maybe $10-15/m ?? (3.8kg/m roughly and $3/kg)
no idea how much the stronger steels will be. call and ask
in a way, it is better to have safties that flex bend, so they absorb energy and hopefulyl stop your bar from bending.
as long as they don't bend enough to ull out of the rack and drop the bar on you
edit: probably mak emore money selling separately, but can take more time depending on what prices you are after..
what cage is it?