We used fiberglass for the 1726 chassis last year and this year. But we did spend some time looking at the kit chassis, and noticed the lowered center holes.
The fiberglass works very well as a chassis material...it's tough, easy to work with if you don't have a machine shop, and the shapes we use work well for robots. The side rails which hold the wheels are I beams turned on their side, the ends are channel, and it's all held together with bolted on aluminum angles (similar to those you've shown above). We learned about this material from our mentor
team 842. We use a
different frame layout than they do, though.