Building Strong, Sturdy Book Crates For Modular Bookcases

I’ve moved twice in the last year, and before I settle down for a while I’ll probably move twice more in the next year. I’ve also got a fair amount of books - gaming books, graphic novels, old textbooks, novels, and even a bunch of boardgames of various sizes. These things are all a frigging pain to move around. I’ve got some bookcases, and I’ve built more than my fair share of cinder block bookshelves, but I have always wanted something more modular that I can just grab and move when my space changes or I decide to rearrange my office. Something that I could section different types of books into so that I could expand or reduce sections as my book collections shift over the years.