In order for them to be effective, they need to be large and rigid. The larger they are, the better. Most woofer cones on speakers are designed around an assumption of infinite baffle, so a smaller enclosure will reduce the output of the lower frequencies.
The biggest thing you need to do is to isolate the backside wave from the frontside wave. Essentially, keep any air volume on the backside of the speaker in its own spot.
The sound pods are actually already decent at this. With a few modifications, they can be made even better. Take some damping sound deadener, the kind with the butyl rubber and foil base (like Dynamat or similar) and stick it on any flat surfaces inside you can reach. This prevents the enclosure from flexing with any significance, and also reduces vibrations. Adding polyfill also significantly slows down the air, making it far more effective. The polyfill also helps absorb higher frequencies by itself, preventing reverberating sound effects from creating peaky response curves. Finally, any obvious air gaps should be sealed. At the seam, at the wire hole, around the light, and especially around the speaker. These don't need to be 100% airtight, but if you were to pressurize the pod, you would want only a very slow rate of air escape.
Essentially, the sound pods are your baffles. They do a great job at it. Car doors on the other hand...
The dash speakers also need some sort of enclosure to effectively work. Unfortunately there's nothing easy to play with except for the sound pod kits.
