Getting back to the OP's topic, I'd suggest going to a packaging/shipping store. Bring the PC, give them UA's maximum allowable dimensions, and tell them what you want to do, including the easy removal from packaging bit for the TSA.

Maybe it's cost $50-100, but they'll figure it out -- I've seen some of them work wonders with weird-sized and fragile stuff that also had to be shipped; since the PC will be carried on it should be pretty easy for them.