You might want to take down the measurements and combined weight of an IATA approved hard travel case and give your friendly neighborhood airline a call to find out what the cost in extra luggage fees will be. You might find out that the cost of the case plus these fees in a couple of round trips might be as much as a new RD700NX,

I'm moving across the country for a few months and I'm trying to figure out whether I should invest in a sturdy travel case for my 700NX and trust that the airline will deliver it in one piece, or whether I should forget that idea and just buy a cheap used piano & sell it before I return.

Has anyone flown with their piano recently and how was the experience? Any recommendations for a travel case?

If you are moving why not ship it? Any of the pack and ship store front shippers can pack a piano. You double box it. It's first rolled in bubble wrap to about a 4" thickness, boxed then that box is put inside a second box with 2" or rigid foam. They might charge you but the cost is MUCH less then an ATA flight case.

One shipper I used once, not for a piano but other large and breakable electronics used foam-in-place charged about $100 for the reusable custom crate.

Buy the case if you intend to routinely ship the piano on airplanes but for a one time deal let Fed Ex ship it. and don't bother with airlines. Let the Fed Ex truck bring it to the new location

After all. Most of these pianos were made in japan, shipped to a warehouse then shipped to you house all in a cardboard box. Boxes do well for a few trips.