i wouldda thought a little section of felt padding right behind where the centre spot is would do the trick, as that's out be the thinnest part of the mirror bearing the most weight. as to how much felt.. it'd be a trial an error thing.. kinda like milk bottle washers. Only thing to be wary of would be stacking the felt too high.. as i wooulda though this would cause astigmatism outwards.. and also.. to keep the felt pieces fairly small.. so to mot effect the eairflow behind the mirror too much.
el
