Take a look at ecrypt-fs, which lets you mount a file system that looks to your OS like a regular directory and is actually backed by another regular directory, except that second regular directory includes cyphertext only. You can have the cyphertext directory live in Dropbox if you want to. (Don't put the unencrypted directory there, naturally, as that results in Dropbox slurping in the plain text.)

When you want to write or read the journal, provide the passphrase and mount it. When you don't, unmount it. You can do this automatically on boot if you're willing to accept the possibility of being tackled by an FBI agent then having your work journal for Silk Road read out in open court.

A second option: use full-disk encryption (same "vulnerable to a football tackle" problem). For backups, just use Tarsnap.