How to attach rebar to a sheet of metal?
Your intentions aren't entirely clear, but here are some general ideas:

U-bolt clamps through holes in the sheet and around the bar
Hose clamps through slots in the sheet and around the bar
Wire through holes in the sheet and around the bar
A friend with a welder (though the dissimilar metals make this challenging)
Bolts through the rebar and the sheet
Screws through the sheet and into the rebar
Wire around the bar from below the sheet to above