I had a custom cover built using the factory cover as a template made out of relatively thin aluminum (forgot the spec sorry) that is vented over the top of the sub to allow it to move some air, but it definitely decreases the output; couple that with a partition and I am down a fair amount of bass in my 01z. But I have got it pretty well tuned that if I really want something to hit I can crank it up about 15db via a remote bass knob and it will hit pretty solidly. Long cruises without major luggage and I will pull the cover and the partition and let the sub do its thing
G'luck
Fej