To be honest, I was just going to use metal epoxy and glue up the ends of some of the 100's of normal rivnuts I have, then put Loctite 243 medium on the bolts. That should be 'IP67' waterproof. I'm more concerned that the rivnuts I have will not work in 5-6 layers of steel sheet! Might have to get some 'long' ones.
Pretty sure a M5 countersunk head bolt is going to have more surface area than a pop rivet head? 10mm outer, 5mm inner diameter, making a ring 2.5mm wide, =58mm2 contact area, and thats not even allowing for the 90Deg angle, which would make it even more. But ah! The material is really thin meaning the inside diameter of the ring will be larger....hmmm.
A 5mm rivet has a head diameter of 9.5mm, creating a surface area of 51.25mm2. Not much in it really...I had 3.2mm rivets in my head. When I went back and read they were 4.8mm rivets, the numbers changed.
But my biggest worry is that to completely flush mount a M5 countersunk, I need 2.8mm of material, including the compressed adhesive tape. So if there's enough clearance for a pop rivet head, I might have to countersink some washers to get me enough clearance so I don't run out of bolt thread to the top of the rivnut before it tightens the track down properly?
Hmmm, maybe rivets are they way to go...thinking a bit different now I know they are
5mm steel rivets