You should look into getting huge blocks of PMMA or PC and have it milled like a metal case (Gotta have that frosted look). The other (more traditional) option is for a "layer cake" case design, but it's less awesome.
The_beast has an excellent chart, above. To answer some materials properties: most polymers can be sanded, but you may or may not want that depending on a number of factors (Surface roughness, opacity, etc). Since they aren't hard, I would imagine most polymers will be easy to machine, but one must keep in mind that sometimes they are brittle (PC for example) and can shatter during machining. It shouldn't corrode with any metal parts (#1 reason you can't have a steel/tungsten (or whatever) case is because it'll corrode because one material is more active).
One thing to consider: With a few exceptions, polymers are high resistivity. This means they make poor shielding for electronics. Most people are not worried about EMF messing up keystrokes or the controller, but you can consider grounding the plate and having some copper foil in the inside (also grounded) to ensure safe operation. This is perhaps overkill for most uses, but the keyboard will perform better inside a faraday cage.
There are several ways to achieve what you want aesthetically and structurally. I'm no architect, but if you want, say, some sort of angle, you can have feet, or a bar on the bottom (to be screwed on) or you can have the bottom be flat (easier to apply rubber friction) and have an internal "bar" that the upper standoffs are mounted to.
As far as construction methods go: you can bolt it together like most metal cases, or you can glue it or (in most cases) "weld" it together either chemically or with heat. There are merits and deficiencies to all methods. If you consider your needs/wants, you can pick what's best.
Lots of options out there! is 2013 "the year of the case" ? I'm excited to see how it turns out.
