Mount it on something flexible to decouple it from the case. There's a product called a Shoggy Sandwich that you can buy, but a few layers of double sided sticky foam will do the job just as well for less money.
well my plan was to make a fibreglass/carbon cover and then bond something like neoprene on the side of it (would be using a flexible PU adhesive like tigerseal to bond it on. i did not really want a huge lump of rubber/neoprene etc sitting there.
this was my idea for the cover
top side of it. wide enough to mount two d5 pumps on
and the under side with the cut out for the sound damping materiel