iMacs actually come with fairly good packaging. I still have the box my 27" Retina iMac came in, and it seems solid. If you have all the original packaging, tape it up and then wrap it in bubble wrap, it will probably be OK.

Alternatively, ship it with a courier service like Fedex and also purchase insurance from them.

Be sure to back up any important data though.

I recently travelled Gla-chc with my 27 inch iMac.

I used the original foam plus more sponge, cloth and bits chucked in with another box(which came with original iMac). Several fragile stickers. Checked in.

Not a problem

Transporting a "27 Mac on EK
 

Hi folks:

Wanted to share my iMac story. I moved from SFO to ME region a few years ago, and was quite nervous transporting my "27 iMac. Shipping and insurance horrendously expensive, so I flew it with me. Bought a Tenba "27 iMac Air Case through B&H Photo after doing extensive research. Yes, the price hurt and I wondered if it was cost effective.

Actual experience flying from SFO to ME, with a night layover was absolutely no problem. Had to admit I imagined all sorts of bad scenarios, and even called Emirates to get reassurance and was told that it would be in secure, locked environment overnight, with CCTV.

At the airport, I had to pay the extra baggage fee since it exceeded dimensions, and saw the baggage attendant walk it over to the special chute for oversize bags. Put a TSA lock on it, and yes, it did get inspected, but it came out at my final destination absolutely fine, with no damage, tampering or pinching of any items. I did add a nice note on the top asking them to please be careful when handling, and thanked them for their service.

The case worked beautifully and is both extremely rugged and easy to maneuver. It's well designed and loads in a flash and even if you never use it for flying, it makes transporting that monster a breeze. The case is actually quite lightweight and has plenty of room for peripherals, but obviously I carried my external hard drive with me.

Cheers