It fits right into these rectangular cutouts on our chassis. The bolts already on the gearbox loosely hold it together while it is on its own so that it does not fall apart, but there are four more bolts that secure it firmly with the chassis.
A student on our team first started designing robots like this for the 2013 season, and I have continued to do the same after he graduated. There isn't much to designing parts like this in CAD. You just have the edge of one part meet the face of another, extrude a row of regularly spaced teeth, and make the corresponding cut on the other part. Then it all comes down to deciding how to position the panels of wood. I haven't seen much in the way of resources for designing for laser cutting, but it wouldn't hurt for our team to make one, now that I think about it.
There are two machines we have used for laser cutting this year. One is an Epilog Helix 24 (18"x24", 60W) which we use for small parts like the gearbox, while the chassis was cut on a Kern HSE100 (100"x52", 400W). (We do not own these machines.)