What is the existing roof rack like for fitting to? is it tubes or bent sections, square or rounded, any tabs for fixing existing floor etc?
Rivents and rivnuts need a flat(ish) plate to fix to and space top drill the holes, so may need tabs welding on to the roof rack for them to work. If you don't plan on removing it often then rivets are a cheap and easy solution and don't take many seconds to remove with a drill if you do want to remove it on the odd occasion. Rivnuts or Dzus fasteners are good if you plan on remoing the sheet often, but don't realy justify the cost/effort if the sheet is never removed.
Another option may be to bond the sheet on using a PU adhesive, though not so good if you plan on being able to remove the sheet.