Rather than use those pods, you can get better bass response by just sealing up all of the holes around where the dash speakers are mounted. That's the real problem. Because the mounting surface isn't sealed off, you get cancellation from the front and rear sound waves coming though the speaker grill. I have pics at home and will post them tonight of how I sealed mine off.
The pods do separate the front and rear sound waves of the speaker, so that helps, but because the internal air volume is so small, it leads to weak mid-bass and peaky bloated upper bass (two things we don't want). If one already has the pods, a good trick would be to put a hole in the back of the pods around an inch in diameter, to allow the speakers to breath better. The rear sound waves would vent through the hole and go towards the firewall, so you wouldn't get the cancellation you do when mounted with nothing blocking it.
Most car speakers are designed for infinite baffle, meaning a large area like in the rear deck of a sedan, or a large door. The pods are just way smaller than the speaker is intended for.