A USB stick with a portable version of Firefox or Chrome (from PortableApps.com).
If you desperately need a portable encrypted email client, consider switching to (which has a somewhat better offline portable experience) or using Mailpile (an open-source email client designed for USB sticks). proton mail desktop app portable
Proton Mail Desktop App Portable: A Complete Guide to Secure Email on the Go A USB stick with a portable version of