You buy new packaging from the speaker manufacturer.
This should 100% be your go-to if the company is still available for it. Barring that, TMR does their own custom injected Styrofoam, molded to the shape of the speaker/component on one side, the intended box on the other, and covered in plastic - that stuff works pretty well, but I’m not sure of its upper limit for size & weight of really big speakers.
Closed-cell foam snugly molded to the speaker ends (optionally with an extra middle piece for tall speakers) is what many manufacturers choose for a reason. Needs to be at least a few inches thick.
This should go without saying, but bubble wrap and packing peanuts and the like are useless here.
Outer box should be double / triple wall cardboard at a minimum and with corner reinforcements for larger suff.