A gun.
For legal purposes, this is a joke, but I'm hoping to illustrate an important point: digital security cannot always compensate for an absence of physical security. If you're in a high-risk space, dont have/use/open a device with sensitive information on it
The best you can do is set screen-lock to a short time period to stymie immediate access, then configure auto-reboot to a short time period to ensure BFU state will be reached in short order. I do not endorse suggestions to grant admin privileges to third-party apps for the purposes of finer lockdown control