Simple way of ensuring a good resin filled base to any rivet in a foam hull is do the following
bend a small nail with about a 5 mm 90 degree bend
drill the rivet or screw size hole through just one skin
put the nail into a small drill
push the 5mm bend of the nail through the drilled hole
give it a whirl with the drill ( you now have a 10mm void made in the foam )
if you want a bigger strengthened area simply put a bent nail with a longer bend on it and give it another whirl. Take small chunks at a time or you could run the risk of damaging the lower skin.
Fill the void with resin and filler ( not micro balloons)
Let the resin set and then drill the correct hole size through both skins. If you are in a hurry you can use one of the fast setting " Araldite " dispensors mixed in with a bit of filler as it sets in about 20 minutes.