I sold my Bassman valve amp a while back and it was packed to within an inch of its life.
I took the valves out that didn't have retainers, wrapped them separately in bubblewrap and stashed them securely back inside the amp case.
Then I wrapped the whole amp with a couple of layers of bubblewrap and put it inside a close-fitting box. Sealed with tape.
That box I then packed into another, much bigger box with the 'prawn cracker' type packing stuff. That gave me two solid layers of protection.
About two hours work, all in. However I knew it would survive anything the courier network could throw at it.
It took the bloke that bought it about twenty minutes to get it out!