Myself and James had the Alan bike box, I was really happy with it. The last box I had was a real struggle getting a 58cm frame in, but this box had no issues.
it is actually cheaper buying through sigma sports than buying direct.
https://www.sigmasports.com/item/Bike-Box-Alan/Bike-Transport-Case-Black/LMG?utm_source=google&utm_medium=base&utm_campaign=base&co=GBR&cu=GBP&glCountry=GB&id=28024&gclid=EAIaIQobChMIzLf6v4fF3QIVgbTtCh10twx3EAQYASABEgJWtvD_BwE&gclsrc=aw.ds