I use a Kinesis Advantage USB ergonomic keyboard. For the last two years and a bit, I've been carrying the keyboard to and from work every day. My only travel case has been the original Kinesis packaging material, reinforced by duct tape, coffee stains, and the rain:
Worst of all, I often travel to customer sites. When I'm on a customer site, I typically use the Kinesis box as a laptop stand. This has not helped the pronounced sag in the cardboard.
I've wanted a hard-sided case for the keyboard since the first time I spilled coffee on the original box. The Kinesis is too wide and too tall for any standard laptop case. I looked into briefcases, but hard-sided briefcases are very expensive, $300-$500. "Halliburton"-style cases are about the same. Lastly, Googling "Kinesis keyboard case" didn't get me anywhere. That is why this blog post exists at all -- I don't want the next traveling Kinesis user to wait as long as I did before buying an appropriate travel case.
The solution I settled on is a Seahorse equipment case, the SE-720. It's much like a Pelican case, but less expensive. At about $80 shipped, it's much less expensive than the Halliburton or briefcase alternatives. Seahorse recommends their products for firearms, fragile DSLRs, etc. Now that I have the case in hand, I can see why. The foam fillers are reasonably sturdy, and the case itself is built like a tank. It seems like it should be a murder weapon in Clue. (The Colonel, in the saloon, with the keyboard case!)
The foam filler is a key advantage to an equipment case over a standard laptop case. It's pre-scored top-to-bottom, sort of like perforated paper:
I was able to create the cavities for the keyboard and cord with just a butter knife to help out. Fuerte Cases, a Seahorse distributor, has a hints page that explains in detail. It took me about 30 minutes to turn the pre-scored foam into a custom insert for the keyboard.
The SE-720 is rather bulky, a little smaller than a typewriter. Surprisingly, after the foam inserts, there's only about 1.5" of extra vertical space over the keyboard. Since the next size down, the SE-710, is about 2" smaller, I don't think it would fit.
2015 update: I ended up buying a SE-710 a few years ago, to replace the 720. It's less bulky, it bangs against my knee less while carrying it, and the keyboard fits just as well.