rclone [0] has great support for B2 [1]. The functionality you want is also available with rclone simply by making a B2 remote then a local encrypted remote which saves to your B2. This will encrypt everything locally before you save it to your B2, and it also has functionality to mount as a system drive. Plenty people do (ahem did [2]) the same with Amazon Cloud Drive's previously unlimited storage.
0: https://rclone.org/ 1: https://rclone.org/b2/ 2: https://www.theregister.co.uk/2017/05/23/amazon_drive_bans_r...