There are other ways to mount things besides using 80/20. If you use a channel section across the front and rear ends, you have a built in mounting and reinforcement panel for the bumpers, as well as a top flange for mounting whatever "stuff" needs to go on top of the chassis. It might not be as easy to use as 80/20, but it also might be stronger in the ways it needs to be if you use something else.
example:
25's robot (and I was quite surprised to see it, as we have the same exact thing on the 1726 robot! pultruded fiberglass C channel)
But you do have to consider what materials and equipment and techniques you have available to construct your robot.