There's a lot of molding and casting techniques that can probably be adapted for your needs. None are as efficient as pro injection molding, but for the midnight engineer...
You can do what movie prop folks do and master vacuum forming. It's quite doable at home, all you need is an oven, a vacuum cleaner, and build yourself a vacuum box. Lots of info on those on Instructables.com and my favorite ones usually end with "... and now you have your Stormtrooper armor."
Generally, it involves a sheet of thermoplastic that's heated up and placed over a positive mold and then the table "vacuums" the plastic to make it readily take the shape of the mold.
Miniature gaming and board gaming folks do something else entirely. They often do stuff with silicone molds and pourable resins and polyurethanes. With a clever "sandwich" of opposing negative molds for each side of your case, you can probably cast some hollow things. The depth limit may not apply, but you'll probably have a little more clean up to do pruning the trees required to pour everything where it needs to go. Nice part is that these use resins which are doable at room temperature and you could wind up with something with more thermal resistance than thermoplastic. One thing to watch for is picking up a resin or polyurethane or whatever that needs "degassing". But there are "degassing-free" chemicals that don't need such elaborate equipment.
The vacu-form process, IMO, is probably the best suited for home building of project cases, as long as you are careful not to make each side too deep, because the plastic will stretch and become thinner the farther it travels. Given my art skills, I'd probably buy a one-off from Shapeways or Pokono and then use that as a master from which to make a negative silicone mold, or do it twice to get a positive mold of form suitable for use with vacuumforming.
If you're ambitious, there's also backyard metalcasting - It's pretty simple to melt aluminum and iron. You can make stamp a model mold of your plastic parts and basically pack in special sand tightly around it to make a mold. Make a few holes, pour the metal in, let it cool (and refinish because it will be a little rough), and then just repeat the process to get a negative metal mold. I guess from there you can work out a way to melt plastic (as to avoid pouring resins which have a much narrower range of physical properties) and use it in the molds as a poor-man's injection molding. It would probably be tough to get the "injection" part of it due to the pressures needed, but, hey, if you're got more time than money...
After all that... I personally love electronics in nice natural wood enclosures.
I like it so much that it's depressing to realize I have no wood working skills whatsoever. But you might.