I’ve been using an Altieri alto flute backpack for several years. It’s certainly held up. I forget where I got the idea, but when I contacted the company that’s what they recommended for clarinet/flute/piccolo, but they also took the dimensions of all my cases. It was very snug on my clarinet case, though I’ve since purchased a protec slim one that fits well. I also keep tons of horn stands in it as well, so it can get crazy heavy. Being a backpack, my other hands are free to carry other horns.
DeMarius Jackson recently posted a video about his doubling case. It looks like a great solution. Possibly a few small downsides, the biggest being the piccolo needs to be in one piece although you could probably keep it in the original case until you need to travel with it. DeMarius is a great follow on YouTube.
What’s your doubling case solution?