Depending on your budget Mono make their own bags that are designed to clip/attach to their own bass gig bag. $$$ but good quality.
Mono is very good quality. I bought 2 gigbags used and they still look like new. Still expensive for “just a bag”
Yes, I have one also and and can fit my B1XFour into one of the medium pockets, cables into the other, music and tablet in the big pocket, and batteries, picks, tools, earplugs and writing instruments into the small pockets.
Oh, yeah, I’ve seen the pictures in the link earlier in the thread. That’s quite some pockets there
But I’m thinking more of non-music-related stuff. In the sense that a couple of trips are coming up and possibly by train instead of car. I wondered if there is a way to reduce suitcase+backpack+gigbag to only suitcase+gigbag. The more stuff that fits in the bag, the better. The bass could end up staying home, it’s not necessary, but it would be nice to have it.
The double bag might be the most useful option in this scenario.
Thank you @John_E @Barney and @Wombat-metal for the suggestions