[DampRobot]

We've used ABS fairly successfully for non-structural electronics boards in the past. However, at the thicknesses that are easy to work with (1/16" is what we usually do), it's essentially useless as a structural member.

We waterjetted some Delrin slides for an elevator this year, which was fairly successful (in terms of the machining quality). ABS and polycarb waterjet nicely too. In fact, it's hard to find something solid that doesn't machine nicely on a waterjet, so that shouldn't be your first concern in terms of your choice of plastics.

You might want to look into high-quality plywood bellypans. They can be made quite thick (and hence rigid) for very low cost. They're also really easy to machine, and can be used with self tapping screws to hold down components. Of course, a nice powdercoated and waterjetted aluminium bellypan might look nicer, but (especially if you're paying for waterjet time) it's a heck of a lot more expensive.