Where do I get custom packaging for my complete .NET Gadgeteer project?
Are you doing the mechanical design yourself?  The first piece of advice would be to hire, or find a mechanical engineer willing to do the case for you.  Plastic is not as simple as just drawing the shape you want. You'll have to decide on things like wall thickness, supports, and watching for sagging.

If you are doing it yourself, I usually use protolabs.  Their cnc service is pretty cheap and can get you just a few parts.   Or their protomold process is a bit more expensive but the parts have a good surface finish.  Plus they help out with the mold design so you don't have to figure out how the plastic will cool and flow in the mold for instance.

Basically if you're after an easy clean professional look you're looking at using injection molding.  If on the other hand you want to put in some work, plenty of protos are made and shown by using a sandable plastic in the 3d printer.

Then with some time and care you can sand it smooth, spray paint it, and maybe put a gloss coat over it.  So many trade show products are made this way ;)