One can run a minimal modal ontological argument for naturalism with just two simple premises:
1. Possibly, there is a necessarily existent extended thing (i.e., the two properties are compossible).
2. What's necessary doesn't vary from possible world to possible world.
3. Therefore, there is a necessarily existent essentially extended thing.
(2) follows from Axiom S5 of S5 modal logic, and most philosophers accept S5, so it's fairly uncontroversial. So the argument comes down to the plausibility of (1). But (1) just says that necessary existence and extension are compossible properties, which seems more plausible than the theistic possibility premise in the corresponding modal ontological argument for theism. For the truth of the latter premise requires acceptance of the compossibility of a large swath of exotic properties, such as omnipotence, omniscience, moral perfection, immateriality, and the capacity for creating individuals and/or stuffs out of nothing. Therefore, it appears that one has more reason to accept the minimal modal ontological argument for naturalism than the standard modal ontological argument for theism.