Well, I’ve never traveled with a harp (or any other instrument for that matter) but here are some recent discussions I’ve happened upon that might help you out:
Flying Commercial Airlines with a Lever Harp
Traveling with harp overseas
I think the general consensus is that traveling with an instrument could be hazardous to your health or your harp’s.

I have a Colorado Case Company fiberglass reinforced flight case for my Dusty Strings FH26 double-strung.

It wasn’t during extreme temperatures and the flights were only a few hours, but I’ve seen messages on the yahoo harp list about people using them on trips to

I have used a Colorado Case flight case for my harps for years.

I bought a Colorado Case with the fiberglass reinforcement for my Christina and was pleased with my first trip from Montreal to South Carolina and back. The low C lever arm did break off after I got back home, but from what I have read on this forum, this is almost to be expected with the low C Camac lever on the Christina and probably nothing to do with the travel – what the Camac levers gain in noiselessness, they also gain in fragility. (And Triplett sent me a replacement with its little wrench at no charge!)
I was pretty impressed that despite significant temperature and humidity changes between the two places, the harp arrived in tune and stayed in tune!
It is more expensive than buying another, smaller travel harp, but I loved having my own harp and all its levers with me, and figured there will be other occasions that I’ll use it for sure!
If you get a Colorado case, do get the fiberglass reinforcement for sure!