I've used 2 different methods
1. Tupperware container in checked luggage. I found one that fit my Aquatica 9.25" dome perfectly.
2. Kata CC-195 bag as carryon. Its a video gear bag but the shape is unique - the bags cross section (height x width) stated as 24x27cm. Perfect for a 23cm dome to sit upright. I can fit my full rig including dome in this bag. The size is questionable as a carry on but i've never had a problem.
**Wow looks like Manfrotto bought Kata. Seems its now the Manfrotto CC-195
Cheers,
Chris