In the Casimir effect, the region between two parallel conducting plates is said to have negative energy density. One of the reason this is interesting is because it seems to violate some energy conditions. Indeed, as shown in an answer to another question, the result for the stress-energy tensor is:
$$ \langle T_{\mu\nu} \rangle_0 = \mathrm{diag}(-1,1,1,-3) \frac{\hbar c}{a^4}\frac{\pi^2}{720} $$
On one hand, the Casimir effect setup produces something relatively simple - just some negative values in the stress-energy tensor. The violation of energy conditions is a rather simple and generic thing. On the other hand, the physical realization of such a setup is rather complicated in comparison - the conducting plates are made out of many particles, which are non-trivially arranged to function as a building blocks of conducting plates. There is a lot of stuff going on.
This seems to indicate that there might be some much simpler setup that could do the same thing.
So the question is: What would be a physical "minimal working example" to achieve this "negativity" (i.e. to achieve $\langle T_{\mu\nu} \rangle_0$ which violates some energy conditions)?