I guess you probably know what I did if you've looked around the gallery, but my box was pretty typical.
Some people just had a base.
Lots of people had a 3 or 4-piece box.
A few people had things like:
356|22/Science Olypmiad Nationals 019.jpg
The year before I started building, the rules had a 50cm^3 box and the people who did ours had trouble reaching in to fix things because it was all packed so closely together (theirs was tiered like the above one). I've always favored the 3-piece box because of that. The two walls and floor gave me enough real estate, but my boxes were all in 75X50X50 years, I think. One of the years might have been 100x50x50 or something, since I remember having to cut my box once. I guess I'll look those up when I have my desktop set up (just got internet today, after moving to new digs).
When you do get around to building your box, whatever design you use, make the base a thick'ish piece of solid wood if you can. I say this for a few reasons:
1) You don't want your base to bend when you pick it up, which could throw things out of alignment.
2) You need something solid to screw things into and to support the sides, so heavy things don't tear loose during transport.
3) You will
spill things on the base. Water, vinegar, whatever...I'd avoid composites and particle boards for the base, since they are likely to absorb spills if left overnight, swell, and warp.
I used plywood for the two sides since it is pretty light and reasonably stiff.
You'll see people using pegboard for their sides:
344|22/Science Olypmiad 015.jpg
Would have made me nervous, since you can see it bending even without a ton of stuff on there.
Super convenient for attaching stuff though.
In 2004, I did use a second tier to store my RCXs and mounted it on a hinge so I could still access the stuff under it easily:
448|29/DarkSabreMission 2004 (17).JPG 454|29/DarkSabreMission 2004 (23).jpg
Wasn't my favorite solution, so I just kept more floor space clear in 2005.