[chris_va]

A thermoelectric generator with a radioactive source would get pretty close.

You can pretty much seal up everything else (encased in a solid glass cube, e.g.). You may want an inert out-gassing source internally, but that would probably only last a few millennia.

[api_or_ipa]

Choosing a trade-off between half-life and radioactive mass would be a fun engineering task. Plutonium-238 has a half-life of 87.7 years and would be long depleted before 55 million years are up. Longer term isotopes would need a ton of mass to get you there.