For the current rules governing Design Patents, refer to MPEP Chapter 16, Section 171:
Whoever invents any new, original and ornamental design for an article
of manufacture may obtain a patent therefor, subject to the conditions
and requirements of this title.
The provisions of this title relating to patents for inventions shall
apply to patents for designs, except as otherwise provided.
As you can see, most of the rules governing Utility Patents also apply to Design Patents. This includes Prior Art and novelty.
Here is excerpt from an old article that sums up the basic requirements for a Design Patent. Note that since this is from 1993, there have been several changes to patent law. However, these basic parameters are still in effect. I couldn't really restate it any better than the original:
Silverman, A. 1993. What Are Design Patents and When Are They Useful? JOM 45(3), p. 63.
To qualify for a design patent, the subject must be new in the sense that no single, identical design exists in the prior art, it must satisfy the ornamental standards, and it must be original to the inventor or inventors seeking protection. It must also be unobvious on the basis of any previously existing design or combination of designs when viewed through the eyes of a hypothetical designer skilled in the art. Further, design patents cannot be obtained for ornamental features that are not visible when the product is in use. In general, a design patent is obtained for the aesthetically appealing features of a product.
Consider the phrase:
an overall design that an ordinary observer would likely find substantially the same
United States Court of Appeals for the Federal Circuit, Case No. 11-CV-1846, May 14, 2012.
which is from the ruling on the Apple vs. Samsung Electronics design patent infringement case. Now, I'm not claiming that this is an official test for novelty of a design patent, but if this is how infringement is tested, then it makes sense that the phrase would also apply to novelty tests of new designs.
Finally, regarding the Nalgene cap, the second figure certainly looks like it incorporates all of the design elements, which means the Nalgene cap should have served as Prior Art for the design. However, further discussion regarding that patent should occur in the original question.