Take heed all whether a long term keeper or someone who wants to preserve some pain free ownership for the next custodian (to me a valuable asset when buying and selling).
The bolts might be ok for a short time but if there are any spire clips involved they will be seized by year 3.
There's a reason why most aftermarket parts suppliers do stainless under tray kits.
I'll be under mine with copper slip and a big can of ACF50
Salt will marmalise that car like it does every lotus to the point where replacing fastenings becomes a significant cost of other repairs.
Both my 111S and Exige were subject to a recall where the floor well panels were either treated with anti corrosion fluid, mats replaced and mat retension bobbins installed or a new floors glued over the originals due to the rubber mats sweating underneath. Salty shoes helping things along.
Elise onwards uses loads of rivnuts. Rivnuts work fine until the rusted bolt threads need more torque than the clamping force of the rivet can endure and the whole thing will spin. In my experience it will spin the rivnut rather than snapping the bolt/screw. Then its out with the drill.........
I have quick release ones on the Exige 'for when I need to check the catalytic converter is still there for the MOT'......