We have a modified design of the nanotubes as our chassis. (We took the CAD file from AndyMark's website and made it longer, got rid of some of the holes, and had it manufactured by one of our sponsors, Vertec Tool, inc.) Personally, as a member of team 2996's drive team, I love the nano tubes. We used 6" Mecanum wheels with it, and made the gear ratio the same as our gear ratio last year when we used 8" Mecanum
Anyways, attaching the bumpers was a problem that took a lot of designing and redesigning to make possible. We ended up having supports that were perpendicular to the nanotubes that extended past the tubes to hold the bumpers. The best picture I could find to show that is
this picture on the manipulator team's blog...the two metal angled aluminum things sticking past the yellow wood is what we attached it to. That and then the bottom was supported by that yellow plywood that was our electronics board!