I also used a bowling ball bag...taking one step back:
For home use, I have a lovely set of Rogue KB's (26, 35, 44, 53, 88) but I didn't really plan on taking any of those on a flight...I did some research and concluded that since people are allowed to take things like bowling balls, golf clubs, etc as checked luggage I could see no reason why a KB should be prohibited. Just in case it was dicey, I bought a cheapo 35-pounder via Craigslist to be my "airline" KB. My thinking on the weight was that most airlines have a weight limit of 50 pounds per checked bag, so the 35-pounder was the right size to include the weight of the bag and some padding around it while staying under the 50 pound limit. Although a 35 is light for me, it's still enough to do some snatches, and you can get creative with some weird exercises like halos and other stuff that I don't normally do.
The bag that I bought has 2 compartments for bowling balls; it stands vertically so one ball would rest on top of the other. Here's how I pack it:
1) Place old towel on the very bottom of bag.
2) Place 35-pound bell in bag in "lower half" compartment.
3) Pack old towels on either side of KB and on top.
4) Secure "lower half" compartment
5) Place shoes, workout clothes, etc in "upper half" of bag
I agree that my bag wouldn't travel very well with just the bell (the towels are needed for some extra support and padding), and the 55-pounder would probably be too heavy. 35 is probably about as heavy of a weight as I would put in the bag and feel good about it; with anything heavier, I would be worried that it might bust something.
I've done this for three or four trips now. Once my bag did get opened for inspection, but it's always arrived on time. I do usually leave a little hand-written note explaining what a kettlebell is along with a picture of me lifting a KB to show that it's a real thing used for exercise...I would love to have seen the TSA agent reaction the first time they opened my bag and saw that...