Foam will be fine when it's covered in doped cloth
Rivets... are a trade secret - but I'll let you in on it!
Swiss graining tool used for gem setting; a set of 16 costs about £10
To start slightly roughen the surface you are going to add the rivet to.
Mix up a blob of greenstuff, you need enough to grip easily in your fingers. I use more yellow than blue to make it sticky.
Place the blob onto the area covering the rivet location and lightly press down.
Moisten the end of the tool (lick)
Use the beading tool to push through the blob and press onto the surface.
Rotate the tool and move it in an orbit (so the edge of the graining tool cuts through the greenstuff)
Remove the tool.
Pull away the blob.
Now you either have a nice raised rivet or a rivet stuck in the tool. My hit rate was about 60%...
With patience you can get some really tiny rivets like the hinges here:
You can also use them to make recessed rivets: just push into the surface and orbit it to cut the surface so t leaves a circle or if you push harder it leaves a recessed dome:
Note free handing the rivet spacing is not a god idea! It's really hard to get them aligned properly and really obvious when you bugger it up! Not for those with OCD...