UniStrut clamped to I beam for under chassis storage

Those could work depending on how tight you can get them. Are you running the Unistrut all the way across the frame, side to side? I'm assuming the pics are cut a ways. Bottom pic would be my choice.

I would juts use some grade 8 3/8" bolts and drill thru the flange. Or weld it to the flange. The angel on the flange could hinder the tightening process.