At a minimum I would say guitar in a case > case in a box > use paper or some sort of filler so it is packed tightly all around. In the past year I started using bags by Mono. They are legit and I like them over my hard cases.
As for an amp I haven't shipped any in a long time, but back when I had to travel from one side of the country to the other pretty often I went out and got this case for my Mark IV. I shipped it, paid way to much to get onto planes and never had an issue.
For amp heads I double box them. Removing tubes is a good idea.
Guitars I usually single box in hard case. Wrap the head and loosen tension on strings. Only had one guitar get damaged in 20 years of shipping (and the one time insurance wasn't applied... That's a story...)
I'm getting a new solid state amp. I mentioned to my ex who is coming to pick me up later this week I needed to buy a smaller/quieter amp. She asked "What about the amp you left here". Hmm I'm getting a new 25w fender solid state apparently I bought years ago to keep at her house to play on. Problem solved lol.
I thought about an attenuater. But from the prices it would have been cheaper to buy a new amp, at least a cheap solid state amp.
The few amps I've bought all had those things made out of heavy cardboard that fit onto each corner to space it out from the box.So that the amp was in the box but 3/4" away from the sides in case something poked thru. Too bad you can't make those to fit.