Disposable per-user VMs ("cows") on my own hardware: 1 GB each, qcow2 copy-on-write overlays, booted to the web in ~12 s. Trial herd is capped at 4.
cow1 (live): visit the running instance
cow1 terminal (live bash): open a shell inside the cow. Sealed sandbox: the cow has no outbound network.
Liveness doctrine: a cow is alive if echo moo succeeds over ssh: cows alive moo.