"moldless" fiberglass/cf construction? Options

[obaa996]

With all the expertise on this list, I thought this might be a good place to start....
I'd like to build a new backpad for my car; I have aftermarket seats, so the stock pad doesn't fit, and besides, I'd like to embed a subwoofer box into it anyway. Since this is going to be a one-off, I don't really want to go through the effort of building a mold, etc. for it either. What I'd like to do is build some sort of form (in the car) which looks like what I want. Hopefully, II can then cover it with fiberglass to form the basic backpad, and then cover it with a layer of carbon fiber (because I like the way it looks). When it's all cured, I'd then remove it from the car, remove the form from behind, and then add more fiberglass to build up the box.
It all sounds good just talking about, but how hard would it be to actually do? What can I make the original form out of? How do I seal/finish off the foam so I can lay fiberglass over it (and be able to remove it later). And how do I get the different fiberglass/cf layers smooth enough so that the finished product will be nice appearance-wise?
All the sites I've found online all discuss building a mold first, but I can't imagine that all the custom pieces I've seen at shows, etc. resulted from a plug/mold/finished part process just for a one-off piece.... Any suggestions here would be appreciated!

[lapuwali]

You've asked enough questions that you'd really need to read a whole book to get all of the answers. There's a good book called "Fiberglass & Composite Materials" (by Forbes Aird, HP Books) which covers the subject nicely in exactly the way you're interested in (small volume manufacture).

As for the problem you're trying to solve, you can just cover the firewall in carpeting. It looks very nice, and leaves you with some extra room behind the seats. Any additional bits, like a subwoofer box, can be made separately using a wide variety of methods and attached to the firewall with brackets through the carpeting. For something like box, using wood or MDF and covering that with matching carpet is very likely going to be much cheaper and easier (and less messy) for one-off construction than doing anything with fiberglass or CF.

If you really want the shiny CF look on the outside of such a box, then you can use a plug mold, which can be as simple as a cardboard box in the correct shape and size. Paint resin onto the outside of the box, then lay CF, then paint more resin over the CF. You'll need to do work on the surface after it cures to get a smooth, even finish by sanding and polishing. You have to do the same thing with a mold, too, as the finish INSIDE the mold is exactly what the surface finish of the final part will look like. The advantage of a mold is you only have to do the surface finish work once, to the mold. The real parts just come out nice (well, mostly).