I've started using a small cheap hardcover notebook that I take with me everywhere. It doesn't have an index, but I use the back pages for lists of ideas, questions, books I see mentioned etc. These will eventually end up in my regular BuJo but there's no urgency..

Because I'm vision-impaired my sidekick is about the same size as my current BuJo but thinner. This side-kick idea means that I don't have to carry my regular BuJo around so I might use a full letter-size book for my next o ne.