My interstate heat shields (below bags) got rattley. And where they touched the exhaust truck stack tips, they corroded a bit (just narrow lines). I took my bags off, and bent the heat shields up (just) a bit in the back. When I put the bags back on, I had to push them down hard to get the bolts tapped (pushing the bent up shields down a little), and every time they come off, I have to do some pushing again. But no more rattle.
Anything like rubber or silicone you stick down there is going to melt, smell and make a mess. A better spacer would be a chunk of steel/nut/washers/small rock, but bending them up a bit in back does the trick.
You are not trying to cushion the heat shields from the stacks, you are moving the shields above the stacks (like only a couple millimeters).
This.^^^^^^^