cut the bottom out of the cabinet so the sub sits on the actual floor, cut vents in the toekick (have cabinet guy do this), dynamat the inside of the cabinet, make sure latch for closing will not rattle if so change it to be more secure (have cabinet guy spec it), if there are glass shelves inthis cabinet with metal supports-felt the bottom of the metal supports,

last but not least explain to the client that this is not the most optimal solution

or you could try this but it may not be in the budget
[Link: jamesloudspeaker.com]