create an encrypted folder, partition or drive on a productive Linux system

Hello, I use an odroid XU4 to control a software RAID 1 under ubuntu 18.04 (arm7). I would like to encrypt the entire drive, create an encrypted partition or an encrypted folder using ecryptfs, EncFS or dm-crypt. Conditions: - the existing data must not be lost, so please discuss the procedure with me (i.e. creating a new partition or folder without touching the existing data, unmounting the RAID, working on one disk first and then to re-establish the RAID) - if a container is used (dm-crypt) it must be dynamic, i.e. should grow if needed - I do not want to encrypt / decrypt individual files. Once opened, I should be able to work with the files normally Rem.: I tried but I keep udnerstand there are problems with FUSE on my odroid. So there is some installation problem to overcome. Thanks for your offers