A couple things:
1. If you can avoid screwing the box together, then don't. Adding screws, let alone adding screws after gluing only makes the joints weaker. Also when using screws, use lengths no longer than 1.5x the sheet's thickness. That means when using .75" mdf, 1 1/4 screws are ideal with 1 1/2 being acceptable but being on the close side. The reason being is the same reasoning behind splitting logs. The longer piece of metal that you put through the middle of that vertical panel, the more stress is trying to split the wood.
2. Resin will not give you any strength by itself. If you meant to fiberglass the whole box, then sure, although that is a monumental waste of time. hardened resin is more brittle than plate glass, which is why it's only used as a binding agent when you are putting mat down with it.
3. Sealing the joints shouldn't be necessary, but if you must, then use silicone sealant. Other caulks and glues can end up weakening the wood which is why glue and sealants are not interchangable. Take this one from me especially. //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif
4. While dowel rods are helpful mainly because they grant you some extra rigidity without compromising air flow, you'll always have better results when you couple more than two sides together. Lattice bracing (that's what I call it) with the enclosure's native material is always the best choice when weight is not an issue. The following image is what I'm talking about on a smaller scale (called shelf bracing, so ignore the speakers, and this isn't my image either):
You want that enclosure to be like it's one solid piece, and coupling four sides together is the second best way to do that, right behind the lattice bracing as shown here:
If weight is an issue, then I agree that threaded rod is a great substitute. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
