My solution to the cat-vs-jenga-shelf problem: put them together permanently. I no longer have access to woodworking tools since my grandfather died, which puts my original modular bookshelf idea out the window. Buuuut, since the friend I'm moving in with has a power drill, and screws are cheap...problem solved! Crates? Check. Drill? Check. Screws? Check. And paint? Check. All stuff I can get my hands on and do without any special instruction ^^