I see the squash ball suggestion is already here :ROFLMAO:
For heavier speakers, racquetballs are the same but bigger.
Granite off cuts are v cheap and they'll even make nice edges for you.
If you fly on Emirates, or some others, you get 2 x 23kg even in economy.
It's probably worth looking at some pro sound equipment cases, they make hard cases that fit amps and such.
I used to occasionally run my motorbike on AvGas. That's 100 octane, leaded. Funnily, it's a lot cheaper than normal petrol as there isn't half the taxes.