In order to use passive cooling, it really helps to have a heatsink that is designed explicitly for passive cooling. For example, passive heatsinks generally have their fins much further apart from one another than heatsinks intended for use with forced air, which makes convective air movement through the fins easier.
So, if you use a DIY solution made explicitly for passive cooling like bluehat
then you can certainly passively cool your CPU. If you want to use a retail solution, I'd say that one of the later model Zalman flowers* would be your best bet, although they're certainly no guarantee of success.
* I believe you'd want the AlCu models rather than the Cu models, since aluminum is supposed to be more effective at dissipating heat than copper, although copper is more effective at absorbing heat. You might want to try spreading the fins out manually on it as well. Zalman's website does mention a flower cooler explicitly designed for cooling C3's passively; if you can find that one for sale, you might do alright with it.