That is the pertinent question - does the inventor hold a patent to the invented prop? If they do, then commercial reproduction would be patent violation. If not, then you're fine from a legal perspective.

That's just flat wrong. What you call "purely philosophical" infringement is often called "de minimis" infringement by courts. That is, just infringing "a little bit" for your own purposes of exploration, and not necessarily for any mass marketing or directly competitive purposes.

The statute that defines infringement is 35 USC 271(a). It reads, in its entirety:


Note that the statute says "whoever ... USES..." When Congress intends to allow de minimis type of exceptions, they will often throw the word "substantial" in there. Like, "whoever ... substantially uses." That word is absent in the patent infringement statute.

In interpreting this statute, the Federal Circuit -- which is second only to the Supreme Court in matters of patent law -- has said:

(quoting Embrex, Inc. v. Service Engineering Corp., 216 F. 3d 1343)

As the court said in its last line, the question of damages is another story. If you make one knock-off of a patented product for your own personal use, that doesn't mean you're on the hook for zillions of dollars in damages. The damage calculation will be very small, in appropriate scope of what your infringement was. It may even be small enough that, as a practical matter, you won't get sued. But your "philosophical use" is still infringement.

I'm not talking about de minimis at all - I'm talking about the permission to use another's patented device for philosophical inquiry, curiosity, or amusement, which, coincidentally, comes from the same Embrex / SEC decision you cite.
"This court has construed both the experimental use and de minimis exceptions very narrowly. See Roche Prods., Inc. v. Bolar Pharm. Co., 733 F.2d 858, 863, 221 USPQ 937, 940 (Fed.Cir.1984) (holding that courts should not "construe the experimental use rule so broadly as to allow a violation of the patent laws in the guise of `scientific inquiry,' when that inquiry has definite, cognizable, and not insubstantial commercial purposes") (superseded on other grounds by 35 U.S.C. § 271(e) (1994)). In Roche, this court considered whether "the limited use of a patented drug for testing and investigation strictly related to FDA drug approval" constituted infringement. Id. at 861. In that case this court recognized: "Section 271(a) prohibits, on its face, any and all uses of a patented invention." Id. Binding precedent from the United States Court of Claims also acknowledged a narrow defense to infringement performed "for amusement, to satisfy idle curiosity, or for strictly philosophical inquiry." Id. at 863; see also Pitcairn v. United States, 212 Ct.Cl. 168, 547 F.2d 1106, 192 USPQ 612 (1976). This court in Roche thus observed that Bolar's alleged experimental use would not even fit within the narrow confines of the alleged experimental use exception because this particular use "in the guise of `scientific inquiry'" had "definite, cognizable, and not insubstantial commercial purposes." Roche, 733 F.2d at 863."