Ok here is a somewhat thorough tutorial and rundown on how to make some A pillar tweeter pods that are simple to make,functional, and clean looking...

You will need:
Respirator
Gloves
1 body filler spreader
Epoxy plastic filler
rubbing alcohol
40-80 and 400 grit sand paper
dremmel tool/razor knife
Spot putty/bondo/glazing putty
texture coat spray
close match to full match spray paint of your interior's color

First off if the tweets you are intending on using have mounting cups definately use them. If for some reason your tweeters don't have mounting cups you will have to make some out of MDF, plywood ,ABS plastic, or you can always try to utilize some tweeter cups from a different brand that will work for your tweets with a little modification or even better yet without any at all. Many stereo shops have spare ones lying around all the time which I'm sure they would let you have for free or a few dollars. So try different ones from the lines of products a local shop carries until you find some that can work...

Here we go: A pillar is removed and a dremmel is used to cut a hole just large enough to squeeze the mounting cup into. You want it just large enough in order to slide the cup in and be able to position and reposition it until you find the angle you want.You want the tweeter cup to be a tight fit so you won't have to worry about it deviating from the angle you chose. It also should be noted that it's easier to get a more uniform look when spreading the filler because the filler isnt dripping into any excess space around the tweeter cup. So try to get your hole cut as accurately and close as possible to the diameter of the tweeter cup. If you cut the hole too large,usually it's no big deal,but this will depend on how much bigger the hole is. However,you will have to use some other means to keep the tweeter cup from falling out of the hole while you are trying to get your angle and to keep it there once you have it.

Rough up around the hole with 40-80 grit sand paper so the filler will adhere better. I also use a razor knife and cut some deep notches around the area where the tweeter cup meets the panel. Once you've done this use some rubbing alcohol to take off any dirt and oil on the panel that would keep the filler from adhering its best.

Now I've got my angle and I'm ready to start filling in the epoxy. Why is my tweeter cup not falling out of the panel? Say it with me! Because you didnt cut the hole too big....Way to go guys!

This is the product I use and it hasnt let me down so far. It's commonly used to repair bumpers and other plastics. It's a rigid hard setting filler and it can be quite a pain in the ass to sand at times ,but it's very durable, it sticks great to plastics, and its strong.

It comes in two tubes.Mix the black with the white in equal amounts and be ready to start filling since the work time is only about 3-5 minutes before it starts to kick and solidify. Wear gloves to keep this crap off your hands. You can usually start sanding it in 10 minutes.

Looks like total crap doesnt it? WEARING your RESPIRATOR, apply the filler and sand away until the filler starts to look more uniform. Once you've done this, apply more filler and sand away. After you have done this, apply more filler and again sand away. Depending on how many times you need to repeat this process is related to how well you are able to apply the filler smoothly and your technique of sanding. I commonly will use 80 grit to sand the filler since the 40 grit seems a bit too abrasive for it. I then go up to either 180 grit or 400 grit.
Keep in mind this stuff doesnt exactly want to spread the smoothest. Even when you are doing thin layers and think that you are applying it as smoothly as possible you will still find it to be pretty wavy in it's texture. Once you feel like you are not going to get it any smoother with either 180-400 grit paper,use a little bit of glazing putty or bondo to get the really small imperfections.

Now we are starting to see some progress. It' finally starting to look like something after about 6-8 layers of filling and sanding. Notice the little dark areas around the tweeter. Those are the little areas of imperfection I mentioned earlier that are so hard to fill in with the epoxy. I was able to fill them in a little bit better with the epoxy ,but I ultimately had to use a little bit of glazing putty to fill these in.

This is after a couple of thin layers of glazing putty were used to fill in the small pits. It sands much easier and spreads smoother. I went up to 400 grit paper.

After you think your work is smooth, feel over it with your bare hand for dips and valleys where you filled in. If you feel any roughness or bumps that your eyes could not see you will definately see them when you paint the panel. So make sure your work is top notch. I sanded the entire A pillar with 400 grit paper to ensure a uniform texture and so that the work I did will blend with the rest of the panel. This is also done so that the texture spray I used would bond to the panel.I used 3 coats of SEM texture spray.
Once the texture spray is dry I sanded it lightly with a little bit of 800 grit to take off any particles.

Finished A pillar pods installed in car. Due that these two pics were shot at night, the unbelievably bright flash my camera has made the color of the pillars look way off from the other panels. They match a good bit better during the day with the human eye. I've thought about wrapping them with vinyl in the future if I can get a color that matches well. You can see behind the Right pillar the old location for the tweeter which didnt provide the best path length equality as the new location.

Well thats it. I hope you will be able to use this tutorial for making your own A pillar pods. Some final things should be mentioned. The more you use this stuff the better you get at it and know how it behaves. Don't try and add too much filler at the start because it will be more work to get off later. I often fall into this trap too,but really try to keep from using too much and keep your layers fairly thin. This takes some time to do ,but it isnt complicated and you arent having to stretch cloth around your enitre A pillar , resin it, sand it, fill it in, and then worry if the A pillar is going to fit like it did orginally. Don't get frustrated and refer back to this tutorial or PM if you have any questions...

Best of luck!