... I'm pretty sure I'd like to stick to boxes ( goofily I still have the box I used last summer, in my garage)
....
I did a van supported trip several years ago with two others that I know in my community, we traveled together to where the trip was run. One of them had a very well built cardboard box that had already seen several trips by BikeFlights, and would probably see several more. He had a hard case at home but for reasons that he did not elaborate on, he liked the cardboard box better than the purpose built case. I suspect it was the perfect size for his bike.
I was going to ship one of my full size bikes by BikeFlights on that ride too, the local Trek store called me when they had a bike box for my bike, when I asked for a bike box they asked what size I needed so they could make sure I got one to fit. Went over and picked up the box and when I went to pack my bike in it, the box was a few inches too short. So, on short notice and lacking a box, I brought my folding bike as checked luggage on the plane on that trip instead.
If your box is the right size and is still in good shape, keep it. I have an old Amtrak box in the basement, almost 10 years old. Not sure if I will ever use it again but if I might need it, I have it.