I built my chassis out of 2 floppy disks, and 2 peanut butter lids for wheels. The floppy's are a bit small but I was able to fit the batteries and servos between them and the bread board on top. The PB lids worked pretty well for wheels, I just poked a tiny hole in the center and then screwed them on to the servos. When I did my first draft everything was held together with poster putty and rubberbands. This worked well for everything but the servos, they need to be secure and not come off, so I ended up using a hot glue gun and attaching them securly to the bottom floppy. The top floppy still uses poster putty which allows me easy access to the batteries.
You will also need to add a caster to the front, basically it just slides around, I rubber banded a rx vial lid to the bottom and it works well on floors (not so well on carpet). Basically making a chassis is up to you, you can salvage parts from the house, the garbage, the kitchen or you can buy them (check out this site for chassis and chassis kits) which might be a bit expensive
There are a couple of downsides to this design
1. Exposure: the circuit and everything else is completely exposed, not good for running into or off things. The robot book I got put all the parts in a sandwich container, much less exposed
2. Wheels: Peanut Butter lids are cheap but you have to make sure your hole is centered (although mine had little dimples in the center) and they're not that sturdy. Check out lego wheels or such is you want better wheels and/or better traction.
