I used stainless setscrews and nuts on the Herald. It's a faff doing some of the ones at the back, but I found it just possible. But I had to do that in some cases because the hole had grown too large, and I needed bigish washers on the inside. And I just couldn't make that work with pop rivets on my own. Maybe with two, one to hold the washer on the rivet end and one to nip it all up, it might have worked.
When I repaired the previous hood poppers with pop rivets, I found a small nut in the bits'n'bobs box that just went on the rivet shank before putting it in the gun. Gave enough extra length on the gun to pop the rivet with the flange at the bottom of the dent in the male popper. Without the nut on there, they just wouldn't go tight.
Graham