Have you considered using Linux’s built-in encryption with LUKS/dm-crypt? It’s widely used for creating encrypted containers on Linux and meets your criteria: open source, audited and Linux-native.
You can create a file container and then use cryptsetup to format it with LUKS. Once set up, you can open the container with a passphrase and mount it as needed. It’s very secure.
For a bit more convenience, there are front-ends like ZuluCrypt that can help manage LUKS containers without getting too deep into the command line if you prefer a GUI.
If you’re comfortable with the terminal, the classic cryptsetup commands give you full control over every step.
Another option is gocryptfs, which offers a user-space encrypted overlay filesystem. It’s open source and lightweight, but the security model is different from LUKS.