Hi,

I’m not familiar with this car but I think it sounds like you need to use something with a large flat head.

My thoughts lean towards the possibility of those black plastic plugs you can find in automotive store.
Volvo puts them all over the 240s and I see them used on todays cars to hold under the fender shields.
They have a stem of various lengths and have fins sticking out around that compress and engage into a hole of sheet metal.

Whatever Volvo used as attachment points to hold it should still be in the firewall. The shield should point to them and apparently it might need more attachment points.
You can add mastic or duct seal compound in addition to the backside of the heat shielding if you are worried about any water or moisture intrusion. Even then, that should be very minimal.

This is a variation of the pop rivet idea, posted earlier, that if used needs to have a bigger head.
This is when I look for a fender washer.

You can eyeball lots of options.

Phil