padding under the rear deck panel?

HI! (First post on this forum - What a great website!)

OK - I 've got a 1997 Mitsubishi Mirage Coupe. I'm redoing the interior and I have recently added "dynamat extreme" style material (foil backed super sticky vibration/sound insulator) to the back deck sheet metal (between the back of the rear seat and glass. I'm going to be putting in 6x9s into a mdf/fg custom "molded" enclosure. So far I've cut out the basic piece that everything else will be mounted to, out of 1/4" MDF using the oem piece as a template and test fit it (so far so good)... here's my two questions:

1. Should I put foam padding (something really thin) under the MDF board - or would the dynamat extreme be enough (see it always remains "squishy" and I'm afraid that bolting the assembled rear deck down could cause the material to flatten out over time) - Or am I just being overly-anal?

2. Should I cut ports into the base of the deck board (allowing the sound to travel into the trunk) or should I leave everything sealed up ?

*Note- Since weight is an issue for me (I'm concerned about stuff like MPG and HP) I don't want to weigh the car down with a big heavy sub-box so I'm planning on using a self powered Bazooka tube for the very-lows. But I still want some tight-punchy-bass so I started thinking that if I leave the 6x9s in a sealed enclosure it may produce a "tighter" fast responding bass (the 6x9s are Sony Xplod 3 way coaxials with an incorporated mid and tweeter in the middle) I've already got a set of Xplod 6.5" round midwoofer in the doors with passive crossover and flush mount tweeter. I plan on adding one 4channel amp and cap.

Thanks in advance for any help! I need some opinions ! :)

For vibrations in a car, I hate the "extreme" type material.  It's got a hard aluminum surface that never get's rid of panel-to-panel rattling.  I see people dump this material in thier car all the time for all the wrong reasons. 

I use a Stinger product called "Overkill."  It has a soft foam pad, like memory foam, and is about a 1/4" thick.  To get rid of panel-to-panel rattling, NOTHING is better.