Notice that the rule and the Q&A both mention "Pool Noodles" though (in quotes), solid or hollow and approximately 2-1/2" Round Petal or Hex. That other link to the McMaster site does not market that round polyethelene as "Pool Noodles." (So are they really "Pool Noodles?")
Why try to 'lawyer' the rules on (of all things), an easily secured material such as "pool noodles?"
And, someone even thinking or posting something like what I saw earlier "They don't open your bumpers during inspection, so don't worry about any violation of a rule"...(It wasn't a direct quote, but could easily be taken that way)...is quite disheartening IMHO, to say the very least. (When the cover rips open in the final rounds of eliminations, District, Regional, or Championships, you are not the only ones directly involved, that may be affected by your teams possible disqualification, and your face might be mighty red! And for what stinking real purpose?)
Inspectors would easily find those "solid steel bars hidden inside as LCOG weight" (or other material), w/ a simple finger / thumb compression squish! Or, hear the "thunk" later, on the field of play."
Please teams, Just Follow the simple rule (choose Solid or Hollow, approx. 2-1/2", a very easily obtained material, and everywhere avail. marketed to be sold as, "Pool Noodles") to simply make your bumpers from.
Go to Walmart.com at the link below (or your local Walmart store, they probably have a few cases right now, stored in the back this time of year and not out on the shelves, if you are in the US snow zone or elsewhere in the cold regions, and may make you a really good deal in the offseason, or better yet (he or she, has a donation budget for good worthwhile community causes, BTW)....Find & tell the Walmart or other store mgr., what you are doing with them exactly (provide them details here), as he / she, may just surprise you quite highly & even donate them to your team, if you request it, to help your school Robotics team out a little financially). Always work this angle...And Get Them "Directly Involved."
http://www.walmart.com/search/?query=Pool%20Noodles
For heavens sake, stay away from the 4" square solid blue ones or the other funky shapes listed at the link above

....Buy the 2.5" round or hex, solid or hollow ones like the rule simply says.
Bumpers Away! 28 days and counting.