You're saying you want to use wood to make a one off mold. Excellent idea. I'll get into that, but first:
The EPS foam pictured, as well as the blue foam known as Styrofoam, will dissolve the moment it comes in contact with the polyester resin one uses with fiberglas, as well as the slightly better/more expensive vinylester. If you used it with a resin that wouldn't dissolve it, such as epoxy, it would destroy the surface of the finished product. You can dissolve EPS, Styrofoam, etc., with the semienvironmentally friendly citric acid. Acetone works better, but that's like a health risk to you as you use it. BUT you can make whatever you want out of the goo, which will harden up solid.
If you want to make a mold, first think draft angle. U is harder to remove your part from than V. The open top could be a half inch bigger than the bottom and it'll be easier to demold. Let's say you use either wood or cardboard (Masking tape has worked well for me) to make your master model of the finished box. First you hotcoat it, take some of the polyester resin and mix in some acetone, all the way to 50:50 if you like, then add your catalyst at 2-3% of your resin. This will soak in well to your wood or cardboard. Sand it, seal it (Primer finisher rather than plain paint) and then release it. PVA (Poly Vinyl Alcohol) or plain release wax. NOT car wax, that has things in it that will activate and STICK.
http://www.rexco-usa.com/waxes-buffing-compound/
http://www.freemansupply.com/PartallFilm10.htm
Among other things, you can cast your mold from plaster. Or instead of making a master model, you make your mold itself from the wood or cardboard and hotcoat, seal and release it as told, then layup inside of it.
When I was a kid I thought the resin itself was fiberglas. I used old cotton diapers as the reinforcement, which worked okay. I used cardboard and resin with old sheets to make tuff boxes, which you could do, except today's sheets might dissolve in polyester resin, haven't tried it lately. Again, hotcoat to provide a good surface the resin can bite when you laminate it. Some spray adhesive to hold edges in place as you laminate works wonders. (I'm still learning my lesson on that one.) http://www.uline.com/BL_8051/Spray-Adhesive
More fun, you could make it out of a castable plastic instead of fiberglas. Perhaps a brush on solid foam urethane right onto your plaster mold. Or you can pour on and swirl the mold around to keep the resin on the sides a few minutes until it sets up. I haven't had a chance to work with this company's "Tuff Stuff" which is different from Mattel "Tuff Stuff." http://www.smooth-on.com/a121/NEW-Flex-Foam-iT!-4-Tuff-Stuff-Pourable-Urethane-Foam/article_info.html
http://www.smooth-on.com
This site has 28 mold making videos, plus others you might find relevant.
http://www.smooth-on.com/media.php
Oh, getting back to that goo from the EPS dissolving, you could make your box out of that, (Got any old coffee cups?) but of course you need something to mold it to.
