I know that what I'm writing about doesn't directly address the topci of "flying with United", but it does address traveling with skis. Given that I live in the Pacific Northwest and we had a terrible snow year, I found myself on an airplane for 4 ski trips this year.
The "Sport Tube" can be found at the following website: http://www.sportube.com
At that site one can find a picture of a Sport Tube and links to dealers. I've used my Sport Tube for 5 different ski trips, all with Southwestern Airlines, and have never had a problem with my skis getting damaged. I paid about $80 or $90 for it.
By the way, I had a shuttle driver (who has been at it for several years) tell me that Southwestern has a very good reputation for getting luggage to the correct destination (they've always done well for me). I figure that a shuttle driver who has talked to a lot of travelers has better insight on this kind of thing than most of us do.
On the topic of locks: There is such a thing as a "TSA" lock. A "TSA" lock is keyed so it can be opened by airport security (they have the keys). A "TSA" lock can be purchased at many places that sell luggage. The TSA locks that I've seen have the letters "TSA" on them, followed by a 3 digit number (the number probably tells airport security which key to use to open it). See http://www.masterlock.com
For myself, I just use a padlock on my Sport Tube. When I check my skis in, I'll tell the people at the counter that I have a lock on my Sport Tube and that I have the key for it. The airport personal will then tell me where to wait in case I need to open it up for airport security. Once airport security is done inspecting my Sport Tube, they'll tell me they're done so I can move on to the gate where my flight is.