But does this app exist? If not, how do you build it? And what are the security trade-offs?
A: Android doesn't have a "portable" concept like Windows. Use the official Proton Mail Android app and a secondary phone.
Will you be plugging this drive into or just your own devices?
: You can find official installers on the Proton Mail Download Page . 2. Unofficial Portable Alternatives
However, as Proton Mail has grown, one specific niche request has emerged from IT professionals, privacy enthusiasts, and "digital nomads":
Use a tool like 7-Zip to open the Proton Mail .exe installer.