I've used encfs for years to keep sensitive data encrypted locally, but I found that with cloud storage, the inability to compress made that approach less than ideal, and the dropbox client refuses to sync the file when it is mounted. The approach I take is to create a standard filesystem, and then reguarly compress and encrypt a copy into the Dropbox folder. The command to do so is below:
cat /overflow/dropbox.ext4 | gzip -f | openssl aes-256-cbc -pass pass:xxx > /overflow/Dropbox/dropbox.ext4.gz.aes-256-cbc
Which I set my backup routine to run when it is finished, but you could also set to run at regular intervals using cron. Occasionaly, you might want to zero out the filesystem to improve compression. This can be done using "cp /dev/zero /mnt/SecureDropbox/zero" followed by "rm /mnt/SecureDropbox/zero"