We build our pit frame out of pvc, not only because it fits the theme of how our chassis and other parts are built out of pvc, but because it's easy for transport. Here's a link to a really good picture of our pit from the Arkansas Regional last year:
[x] It also has some good shots of the two neighboring pits as well. I would definitely invest in the foam tile flooring, it comes in really well. GeeTwo mentioned a list of plenty of really good suggestions to keep in mind.
Another set of things that you should keep in mind is safety for sure. There will be safety judges, inspectors, and many other people waking by on monitor for good safety, as well as getting a review slip both Thursday and Friday night. Keep things like a first aid kit, fire extinguisher, battery spill kit (baking soda, acid resistant gloves, a disposal container, etc.). It would also be good to have extra safety glasses around in case someone forgets their pair, but the event will provide pairs even though they're mostly for non-team visitors. Things like extra hair ties and stuff like that. The biggest safety thing would be an MSDS (Material Safety Data Sheet) of all of the chemicals you use and even the main breaker switch sticker for the robot.
There are resources like the
2015 Safety Manual that can help you out with all of that!
Good luck at competition!