I used to use BoxCyptor to do Dropbox encryption, i always was a pain to install, but worked well.
Not sure if this applies to Safemonk, but be aware of any app that requests your dropbox username/password combo to carry out any activities. One of the big hacks recently has been via small apps/websites that use your details to log in and carry out activities on other sites as they don't always have the same level of security and updates of their software and servers as the big companies do. This was how the Snapchat hack happened.
Taff