encryption of mounted usbdisk topic

Hi, been searching this morning for a way of encrypting either the whole drive or individual files on a usbdisk mounted via OTG cable. Had no luck hence asking here...

Phone is encrypted, but I mount a USB pendrive for backing up, need a way to encrypt the content on the pendrive.

I have tried secrets from fdroid repo and configured it to store vaults on mounted usbdisk but it crashes out on files or more than a few mb.

Anyone got a solution (no access to play store)