We use Tuffak polycarbonate (made by one of our sponsors!). It's very sturdy, easy to machine and shape and can support a reasonable amount of weight. We drill and bolt/nut the components to the Tuffak. The clear nature of the material lets us look through it to other components that may be mounted on the other side. It's light, strong, an insulator and has worked well for the last 5 robots.
We've talked about Al's zip tie method, but our team has gotten better at doing design first, drilling holes to the nearest 32'nd to make the mounting boards possible. On the other hand, we do zip tie things to the frame during the mockup process. There is a lot to be said for the "place, DoH!, snip and move process." Bonus is you can find zip ties in your team colors and make it part of the robot bling.
Don't forget to insulate the cRio and the camera from the frame!
This is a "before picture" showing temp mounting of the digital side car using zip ties. You can see the Jags on each side mounted on their boards. It is also the poster picture of how to NOT to wire a robot. (All the cables have been dressed and once I get back into the shop I'll post a picture on the improved look.)