Whatever you choose, make sure you get "file by file" encryption not "container" encryption like truecrypt. Using "container" encryption requires re-sync of the entire container even if you just change 1 file. Thus, products like Cloudfogger or BoxCryptor are absolutely necessary.
BoxCryptor -
https://www.boxcryptor.com/
CloudFogger -
http://www.cloudfogger.com/en/download/
Cloudfogger is free...