The new (bigger) magnets did the trick, while still being small enough that I think they don’t upset the look of the journals, and I made a couple without magnets and used leather straps to hold them closed.
My original design didn’t work with the wood I was using. I used the same cypress I use for my books. It’s the same stuff I buy from the hardware store, and it splits a lot. My design called for two big dove tail joints to connect the main cross beam at the top to the uprights at the side. Appart from my joinery being sloppy, the wood split and the join just won’t hold.