Apps similar to Cryptmount include Hat.sh, which is free and open source. Other options are Tomb. Between them they cover Mac, Windows, Linux and the web.
hat.sh is a javascript app that provides secure file encryption using the AES-256-GCM algorithm from WebCryptoAPI provided by your browser. it was coded following the WebCrypto Documentations .
Tomb is an 100% free and open source system for file encryption on GNU/Linux, facilitating the backup of secret files. Tomb is written in code that is easy to review and links commonly shared components.